From Wikipedia, the free encyclopedia
An overview of how VoIP works
A typical analog telephone adapter for connecting an ordinary phone to a VoIP network
Voice over Internet Protocol, also called VoIP, IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband is the routing of voice conversations over the Internet or through any other IP-based network.
Companies providing VoIP service are commonly referred to as providers, and protocols which are used to carry voice signals over the IP network are commonly referred to as Voice over IP or VoIP protocols. They may be viewed as commercial realizations of the experimental Network Voice Protocol (1973) invented for the ARPANET providers. Some cost savings are due to utilizing a single network - see attached image - to carry voice and data, especially where users have existing underutilized network capacity that can carry VoIP at no additional cost. VoIP to VoIP phone calls are sometimes free, while VoIP to PSTN may have a cost that's borne by the VoIP user.
There are two types of PSTN to VoIP services: DID (Direct Inward Dialing) and access numbers. DID will connect the caller directly to the VoIP user while access numbers require the caller to input the extension number of the VoIP user. Access numbers are usually charged as a local call to the caller and free to the VoIP user while DID usually has a monthly fee. There are also DIDs that are free to the VoIP user but chargeable to the caller.
VoIP can facilitate tasks that may be more difficult to achieve using traditional networks:
- Incoming phone calls can be automatically routed to your VoIP phone, regardless of where you are connected to the network. Take your VoIP phone with you on a trip, and wherever you connect to the Internet, you can receive incoming calls.
- Free phone numbers for use with VoIP are available in the USA, UK and other countries from organizations such as VoIP User.
- Call center agents using VoIP phones can work from anywhere with a sufficiently fast and stable Internet connection.
- Many VoIP packages include PSTN features that most telcos (telecommunication companies) normally charge extra for, or may be unavailable from your local telco, such as 3-way calling, call forwarding, automatic redial, and caller ID.
VoIP allows users to travel anywhere in the world and still make and receive phone calls:
- Subscribers of phone-line replacement services can make and receive local phone calls regardless of their location. For example, if a user has a New York City phone number and is traveling in Europe and someone calls the phone number, it will ring in Europe. Conversely, if a call is made from Europe to New York City, it will be treated as a local call. Of course, there must be a connection to the Internet e.g. WiFi to make all of this possible.
- Users of Instant Messenger based VoIP services can also travel anywhere in the world and make and receive phone calls.
- Specialized mobile VoIP services enable users to talk over internet protocol and use VoIP based Instant Messenger services like Skype, MSN and GoogleTalk from their mobile phones.
- VoIP phones can integrate with other services available over the Internet, including video conversation, message or data file exchange in parallel with the conversation, audio conferencing, managing address books and passing information about whether others (e.g. friends or colleagues) are available online to interested parties.
Difficulty with sending faxes
One drawback is the difficulty in sending faxes due to software and networking constraints in most home systems. An effort is underway to remedy this by defining an alternate IP-based solution for delivering Fax-over-IP, namely the T.38 protocol. Another possible solution to overcome the drawback is to treat the fax system as a message switching system which does not need real time data transmission - such as sending a fax as an email attachment (see iFax) or remote printout (see Internet Printing Protocol). The end system can completely buffer the incoming fax data before displaying or printing the fax image.
Internet connection requirement
Another drawback of VoIP service is its reliance upon another separate service - an Internet connection. The quality and overall reliability of the phone connection is entirely reliant upon the quality, reliability, and speed of the Internet connection which it is using. Shortcomings with Internet connections and Internet Service Providers (ISPs) can affect VoIP calls. Higher overall network latencies can lead to significantly reduced call quality and cause certain problems such as echoing.
VoIP is not entirely reliant upon internet connections, however. VoIP systems can also utilize regular telephone lines and business-grade connections like T1 for voice service.
A few business VoIP providers offer dedicated point-to-point T1 connections, not relying on an Internet connection for service. Although residential VoIP service typically uses only an Internet connection, business-grade VoIP service can use a variety of connection methods to provide ongoing phone service.
Many VoIP users still maintain a traditional analog voice line (business line) which allows them to dial emergency numbers and utilize a traditional fax machine.
Another drawback of VoIP is the likely inability to make phone calls during a power outage, though this problem is shared with some conventional phones which require more power than the landline itself can supply. Cordless phone units in particular are more affected by power outages as many do not have a battery backup option for the base station. This can be remedied with a battery backup like an Uninterruptible Power Supply. During a power outage one also has the choice to forward calls to a cell phone or alternate number.
If VoIP is used in solitary LAN (with no Internet connection), it would consume more resources compared to a PABX.
Modems are now available with lithium ion battery backup so that you can use the service with no external power.
Because UDP does not provide a mechanism to ensure that data packets are delivered in sequential order, or provide Quality of Service guarantees, VoIP implementations face problems dealing with latency and jitter. This is especially true when satellite circuits are involved, due to long round trip propagation delay (400 milliseconds to 600 milliseconds for geostationary satellite). The receiving node must restructure IP packets that may be out of order, delayed or missing, while ensuring that the audio stream maintains a proper time consistency. This functionality is usually accomplished by means of a jitter buffer.
Another challenge is routing VoIP traffic through firewalls and address translators. Private Session Border Controllers are used along with firewalls to enable VoIP calls to and from a protected enterprise network. Skype uses a proprietary protocol to route calls through other Skype peers on the network, allowing it to traverse symmetric NATs and firewalls. Other methods to traverse firewalls involve using protocols such as STUN or ICE.
- Delay/Network Latency
- Packet loss
Fixed delays cannot be controlled but some delays can be minimized by marking voice packets as being delay-sensitive (see, for example, Diffserv).
The principal cause of packet loss is congestion, which can be controlled by congestion management and avoidance. Carrier VoIP networks avoid congestion by means of traffic engineering.
Variation in delay is called jitter. The effects of jitter can be mitigated by storing voice packets in a buffer (called a play-out buffer) upon arrival, before playing them out. This avoids a condition known as buffer underrun, in which the playout process runs out of voice data to play because the next voice packet has not yet arrived, but increases delay by the length of the buffer.
Common causes of echo include impedance mismatches in analog circuitry, and acoustic coupling of the transmit and receive signal at the receiving end.
Conventional phones are connected directly to telephone company phone lines, which in the event of a power failure are kept functioning by back-up generators or batteries located at the telephone exchange. However, household VoIP hardware uses broadband modems and other equipment powered by household electricity, which may be subject to outages dictating the use of an uninterruptible power supply or generator to ensure availability during power outages. Early adopters of VoIP may also be users of other phone equipment, such as PBX and cordless phone bases, that rely on power not provided by the telephone company. Even with local power still available, the broadband carrier itself may experience outages as well. While the PSTN has been matured over decades and is typically extremely reliable, most broadband networks are less than 10 years old, and even the best are still subject to intermittent outages. Furthermore, consumer network technologies such as cable and DSL often are not subject to the same restoration service levels as the PSTN or business technologies such as T-1 connection.
Quality of Service
Some broadband connections may have less than desirable quality. Where IP packets are lost or delayed at any point in the network between VoIP users, there will be a momentary drop-out of voice. This is more noticeable in highly congested networks and/or where there is long distances and/or interworking between end points. Technology has improved the reliability and voice quality over time and will continue to improve VoIP performance as time goes on.
The nature of IP makes it difficult to geographically locate network users. Emergency calls, therefore, cannot easily be routed to a nearby call center, and are impossible on some VoIP systems. Sometimes, VoIP systems may route emergency calls to a non-emergency phone line at the intended department.
In the US, at least one major police department has strongly objected to this practice as potentially endangering the public.
Moreover, in the event that the caller is unable to give an address, emergency services may be unable to locate them in any other way. Following the lead of mobile phone operators, several VoIP carriers are already implementing a technical work-around. For instance, one large VoIP carrier requires the registration of the physical address where the VoIP line will be used. When you dial the emergency number for your country, they will route it to the appropriate local system. They also maintain their own emergency call center that will take non-routable emergency calls (made, for example, from a software based service that is not tied to any particular physical location) and then will manually route your call once learning your physical location.
The United States government had set a deadline, requiring VoIP carriers to implement E911; however, the deadline is being appealed by several of the leading VoIP companies.
This is a different situation with IPBX systems, where these corporate systems often have full E911 capabilities built into the system.
Integration into global telephone number system
While the traditional Plain Old Telephone System (POTS) and mobile phone networks share a common global standard (E.164) which allocates and identifies any specific telephone line, there is no widely adopted similar standard for VoIP networks. Some allocate an E.164 number which can be used for VoIP as well as incoming/external calls. However, there are often different, incompatible schemes when calling between VoIP providers which use provider specific short codes.
Single point of calling
With hardware VoIP solutions it is possible to connect the VoIP router into the existing central phone box in the house and have VoIP at every phone already connected. Software based VoIP services require the use of a computer, so they are limited to single point of calling, though handsets are now available, allowing them to be used without a PC. Some services provide the ability to connect WiFi SIP phones so that service can be extended throughout the premises, and off-site to any location with an open hotspot. However, note that many hotspots require browser-based authentication, which most SIP phones do not support.
Mobile phones & Handheld Devices
Telcos and consumers have invested billions of dollars in mobile phone equipment. In developed countries, mobile phones have achieved nearly complete market penetration, and many people are giving up landlines and using mobiles exclusively. Given this situation, it is not entirely clear whether there would be a significant higher demand for VoIP among consumers until either public or community wireless networks have similar geographical coverage to cellular networks (thereby enabling mobile VoIP phones, so called WiFi phones) or VoIP is implemented over legacy 3G networks. However, "dual mode" handsets, which allow for the seamless handover between a cellular network and a WiFi network, are expected to help VoIP become more popular.
The first mobile VoIP solutions were fring and Truphone. Phones like the Nokia E60, E61 have been the first "dual mode" handsets capable of delivering mobile VoIP with long battery lifetimes. With more and more mobile phones and handheld devices using VOIP, the nicknames of "MoIP" and MVoip (Mobile VoIP)have been attributed to these mobile applications.
Handheld Devices are another type of medium whereby you can use VoIP services. Since most of these devices are limited to using GSM/GPRS type of communication mediums, almost all of the handheld devices use WiFi of some sort.
The majority of consumer VoIP solutions do not support encryption yet. As a result, it is relatively easy to eavesdrop on VoIP calls and even change their content. There are several open source solutions that facilitate sniffing of VoIP conversations. A modicum of security is afforded due to patented audio codecs that are not easily available for open source applications, however such security through obscurity has not proven effective in the long run in other fields. Some vendors also use compression to make eavesdropping more difficult. However, real security requires encryption and cryptographic authentication which are not widely available at a consumer level. The existing secure standard SRTP and the new ZRTP protocol is available on Analog Telephone Adapters(ATAs) as well as various softphones. It is possible to use IPsec to secure P2P voip by using opportunistic encryption. A notable softphone which does not use SRTP but uses strong encryption by default is Skype.
The Voice VPN solution provides secure voice for enterprise VoIP networks by applying IPSec encryption to the digitized voice stream.
Pre-Paid Phone Cards
VoIP has become an important technology for phone services to travellers, migrant workers and ex-pats, who either, due to not having a fixed or mobile phone or high overseas roaming charges, choose instead to use VoIP services to make their phone calls. Pre-paid phone cards can be used either from a normal phone or from Internet cafes that have phone services. Developing countries and areas with high tourist or immigrant communities generally have a higher uptake.
Caller ID support among VoIP providers varies, although the majority of VoIP providers now offer full Caller ID w/ Name on outgoing calls. When calling a traditional PSTN number from some VoIP providers, Caller ID isn't supported, and therefore the target person will not know who is calling. The number shows up as 'Unknown' or '000-012-3456'.
In a few cases, VoIP providers may allow a caller to spoof the Caller ID information, making it appear as though they are calling from a different number. Business grade VoIP equipment and software often makes it easy to modify caller ID information. Although this can provide many businesses great flexibility, it is also open to abuse.
A major development starting in 2004 has been the introduction of mass-market VoIP services over broadband Internet access services, in which subscribers make and receive calls as they would over the PSTN. Full phone service VoIP phone companies provide inbound and outbound calling with Direct Inbound Dialing. Many offer unlimited calling to the U.S., and some to Canada or selected countries in Europe or Asia as well, for a flat monthly fee.
These services take a wide variety of forms which can be more or less similar to traditional POTS. At one extreme, an analog telephone adapter (ATA) may be connected to the broadband Internet connection and an existing telephone jack in order to provide service nearly indistinguishable from POTS on all the other jacks in the residence. This type of service, which is fixed to one location, is generally offered by broadband Internet providers such as cable companies and telephone companies as a cheaper flat-rate traditional phone service. Often the phrase "VoIP" is not used in selling these services, but instead the industry has marketed the phrase "Internet Phone" or "Digital Phone" which is aimed at typical phone users who are not necessarily tech-savvy. Typically, the provider touts the advantage of being able to keep one's existing phone number.
At the other extreme are services like Gizmo Project and Skype which rely on a software client on the computer in order to place a call over the network, where one user ID can be used on many different computers or in different locations on a laptop. In the middle lie services which also provide a telephone adapter for connecting to the broadband connection similar to the services offered by broadband providers (and in some cases also allow direct connections of SIP phones) but which are aimed at a more tech-savvy user and allow portability from location to location. One advantage of these two types of services is the ability to make and receive calls as one would at home, anywhere in the world, at no extra cost. No additional charges are incurred, as call diversion via the PSTN would, and the called party does not have to pay for the call. For example, if a subscriber with a home phone number in a U.S. area code calls someone else in his home area code, it will be treated as a local call regardless of where that person is in the world. Often the user may elect to use someone else's area code as his own to minimize phone costs to a frequently called long-distance number.
For some users, the broadband phone complements, rather than replaces, a PSTN line, due to a number of inconveniences compared to traditional services. VoIP requires a broadband Internet connection and, if a telephone adapter is used, a power adapter is usually needed. In the case of a power failure, VoIP services will generally not function. Additionally, a call to the U.S. emergency services number 9-1-1 may not automatically be routed to the nearest local emergency dispatch center, and would be of no use for subscribers outside the U.S. This is potentially true for users who select a number with an area code outside their area. Some VoIP providers offer users the ability to register their address so that 9-1-1 services work as expected.
Another challenge for these services is the proper handling of outgoing calls from fax machines, TiVo/ReplayTV boxes, satellite television receivers, alarm systems, conventional modems or FAXmodems, and other similar devices that depend on access to a voice-grade telephone line for some or all of their functionality. At present, these types of calls sometimes go through without any problems, but in other cases they will not go through at all. And in some cases, this equipment can be made to work over a VoIP connection if the sending speed can be changed to a lower bits per second rate. If VoIP and cellular substitution becomes very popular, some ancillary equipment makers may be forced to redesign equipment, because it would no longer be possible to assume a conventional voice-grade telephone line would be available in almost all homes in North America and Western-Europe. The TestYourVoIP website offers a free service to test the quality of or diagnose an Internet connection by placing simulated VoIP calls from any Java-enabled Web browser, or from any phone or VoIP device capable of calling the PSTN network.
Corporate and telco use
Although few office environments and even fewer homes use a pure VoIP infrastructure, telecommunications providers routinely use IP telephony, often over a dedicated IP network, to connect switching stations, converting voice signals to IP packets and back. The result is a data-abstracted digital network which the provider can easily upgrade and use for multiple purposes.
Corporate customer telephone support often use IP telephony exclusively to take advantage of the data abstraction. The benefit of using this technology is the need for only one class of circuit connection and better bandwidth use. Companies can acquire their own gateways to eliminate third-party costs, which is worthwhile in some situations.
VoIP is widely employed by carriers, especially for international telephone calls. It is commonly used to route traffic starting and ending at conventional PSTN telephones.
Many telecommunications companies are looking at the IP Multimedia Subsystem (IMS) which will merge Internet technologies with the mobile world, using a pure VoIP infrastructure. It will enable them to upgrade their existing systems while embracing Internet technologies such as the Web, email, instant messaging, presence, and video conferencing. It will also allow existing VoIP systems to interface with the conventional PSTN and mobile phones.
Electronic Numbering (ENUM) uses standard phone numbers (E.164), but allows connections entirely over the Internet. If the other party uses ENUM, the only expense is the Internet connection.
Use in Amateur Radio
Amateur radio has adopted VoIP by linking repeaters and users with Echolink, IRLP, Dstar and EQSO. Echolink and IRLP are programs/systems based upon the Speak Freely VoIP open source software. In fact, Echolink allows users to connect to repeaters via their computer (over the internet) rather than by using a radio. By using VoIP Amateur Radio operators are able to create large repeater networks with repeaters all over the world where operators can access the system with actual ham radios.
Ham Radio operators using radios are able to tune to repeaters with VoIP capabilities and use DTMF buttons to command the repeater to connect to various other repeaters, thus allowing them to talk to people all around the world, however powerful their radio. Dingotel offers a similar feature for non ham radio users by providing a P2P network to link FRS radios.
Click to call
Click-to-call is a service which lets users click a button and immediately speak with a customer service representative. The call can either be carried over VoIP, or the customer may request an immediate call back by entering their phone number. One significant benefit to click-to-call providers is that it allows companies to monitor when online visitors change from the website to a phone sales channel.
Legal issues in different countries
As the popularity of VoIP grows, and PSTN users switch to VoIP in increasing numbers, governments are becoming more interested in regulating VoIP in a manner similar to legacy PSTN services, especially with the encouragement of the state-mandated telephone monopolies/oligopolies in a given country, who see this as a way to stifle the new competition.
In the U.S., the Federal Communications Commission now requires all VoIP operators who do not support Enhanced 911 to attach a sticker warning that traditional 911 services aren't available. The FCC recently required VoIP operators to support CALEA wiretap functionality. The Telecommunications Act of 2005 proposes adding more traditional PSTN regulations, such as local number portability and universal service fees. Other future legal issues are likely to include laws against wiretapping and network neutrality.
Some Latin American and Caribbean countries, fearful for their state owned telephone services, have imposed restrictions on the use of VoIP, including in Panama where VoIP is taxed. In Ethiopia, where the government is monopolizing telecommunication service, it is a criminal offence to offer services using VoIP. The country has installed firewalls to prevent international calls being made using VoIP. These measures were taken after a popularity in VoIP reduced the income generated by the state owned telecommunication company.
In the European Union, the treatment of VoIP service providers is a decision for each Member State's national telecoms regulator, which must use competition law to define relevant national markets and then determine whether any service provider on those national markets has "significant market power" (and so should be subject to certain obligations). A general distinction is usually made between VoIP services that function over managed networks (via broadband connections) and VoIP services that function over unmanaged networks (essentially, the Internet).
VoIP services that function over managed networks are often considered to be a viable substitute for PSTN telephone services (despite the problems of power outages and lack of geographical information); as a result, major operators that provide these services (in practice, incumbent operators) may find themselves bound by obligations of price control or accounting separation.
VoIP services that function over unmanaged networks are often considered to be too poor in quality to be a viable substitute for PSTN services; as a result, they may be provided without any specific obligations, even if a service provider has "significant market power".
The relevant EU Directive is not clearly drafted concerning obligations which can exist independently of market power (e.g., the obligation to offer access to emergency calls), and it is impossible to say definitively whether VoIP service providers of either type are bound by them. A review of the EU Directive is under way and should be complete by 2007.
In India, it is legal to use VoIP, but it is illegal to have VoIP gateways inside India. This effectively means that people who have PCs can use them to make a VoIP call to any number, but if the remote side is a normal phone, the gateway that converts the VoIP call to a POTS call should not be inside India.
In the UAE, it is illegal to use any form of VoIP, to the extent that websites of Skype and Gizmo Project don't work.
In the Republic of Korea, only providers registered with the government are authorized to offer VoIP services. Unlike many VoIP providers, most of whom offer flat rates, Korean VoIP services are generally metered and charged at rates similar to terrestrial calling. Foreign VoIP providers such as Vonage encounter high barriers to government registration. This issue came to a head in 2006 when internet service providers providing personal internet services by contract to United States Forces Korea members residing on USFK bases threatened to block off access to VoIP services used by USFK members of as an economical way to keep in contact with their families in the United States, on the grounds that the servicemembers' VoIP providers were not registered. A compromise was reached between USFK and Korean telecommunications officials in January 2007, wherein USFK servicemembers arriving in Korea before June 1, 2007 and subscribing to the ISP services provided on base may continue to use their U.S.-based VoIP subscription, but later arrivals must use a Korean-based VoIP provider, which by contract will offer pricing similar to the flat rates offered by U.S. VoIP providers. 
IP telephony in Japan
In Japan, IP telephony (IP電話, IP Denwa ?) is regarded as a service applied VoIP technology to whole or a part of the telephone line. As from 2003, IP telephony service assigned telephone numbers has been provided. There are not voice only services, but also videophone service. According to the Telecommunication Business Law, the service category for IP telephony also implies the service provided via Internet, which is not assigned any telephone number. IP telephony is basically regulated by Ministry of Internal Affairs and Communications (MIC), as a telecommunication service. The operators have to disclose necessary information on its quality, etc, prior to making contract with customers, and have obligation to respond to their complaints cordially.
Many Internet service providers (ISP) are providing IP telephony services. The provider, which provides IP telephony service, is so-called "ITSP (Internet Telephony Service Provider)". Recently, the competition among ITSPs has been activated, by option or set sales, connected with ADSL or FTTH services.
The tariff system normally applied for Japanese IP telephony tends to be described as below;
- The call between IP telephony subscribers, limited to the same group, is mostly free of charge.
- The call from IP telephony subscribers to fixed line or PHS is mostly fixed rate, uniformly, all over the country.
Between ITSP, the interconnection is mostly maintained at VoIP level.
- As for the IP telephony assigned normal telephone number (0AB-J), the condition for its interconnection is considered same as normal telephony.
- As for the IP telephony assigned specific telephone number (050), the condition for its interconnection tends to be described as below;
- Interconnection is sometimes charged. (Sometimes, it's free of charge.) In case of free of charge, mostly, the traffics are exchanged via P2P connection with the same VoIP standard. Otherwise, certain conversion is needed at the point of VoIP gateway, which needs running costs.
Telephone number for IP telephony in Japan
Since September, 2002, MIC has assigned IP telephony the telephone numbers on the condition that the service falls into the certain categories of quality requirement. The high qualified IP telephony is assigned the telephone number. Normally the number starts with 050. But, when its quality is so high that customer almost could not tell the difference between normal telephone and when the provider relates its number with the location and provides the connection with emergency call, it is allowed to assign normal telephone number, which is so-called "0AB-J" number.
The two major competing standards for VoIP are the IETF standard SIP and the ITU standard H.323. Initially H.323 was the most popular protocol, though in the "local loop" it has since been surpassed by SIP. This was primarily due to the latter's better traversal of NAT and firewalls, although recent changes introduced for H.323 have removed this advantage.
However, in backbone voice networks where everything is under the control of the network operator or telco, H.323 is the protocol of choice. Many of the largest carriers use H.323 in their core backbones
, and the vast majority of callers have little or no idea that their POTS calls are being carried over VoIP.
Where VoIP travels through multiple providers' softswitches the concepts of Full Media Proxy and Signalling Proxy are important. In H.323, the data is made up of 3 streams of data: 1) H.225.0 Call Signaling; 2) H.245; 3) Media. So if you are in London, your provider is in Australia, and you wish to call America, then in full proxy mode all three streams will go half way around the world and the delay (up to 500-600 ms) and packet loss will be high. However in signaling proxy mode where only the signaling flows through the provider the delay will be reduced to a more user friendly 120-150 ms.
One of the key issues with all traditional VoIP protocols is the wasted bandwidth used for packet headers. Typically, to send a G.723.1 5.6 kbit/s compressed audio path requires 18 kbit/s of bandwidth based on standard sampling rates. The difference between the 5.6 kbit/s and 18 kbit/s is packet headers. There are a number of bandwidth optimisation techniques used, such as silence suppression and header compression. This can typically save 35% on bandwidth usage. But the really interesting technology comes from VoIP offshoots such as TDMoIP which take advantage of the concept of bundling conversations that are heading to the same destination and wrapping them up inside the same packets. These can offer near toll quality audio in a 6-7 kbit/s data stream.
VOIP traffic (along with video traffic etc.) is called purple minutes.
- List of commercial voice over IP network providers
- IP Multimedia Subsystem
- Mobile VoIP
- Comparison of VoIP software
- Computer conferencing
- Differentiated services
- Integrated services
- Predictive dialers
- Secure telephone
- SIP Trunking
- Voip recording
- ^ VoIP at the BPC Glossary
- ^ PC World's VoIP cost comparisons
- ^ Internet Telephony Services Providers’ Association, "Emergency Services – Technical Issues, Potential Solutions and Benefits", p9-10
- ^ letter from the City of New York to the Federal Communications Commission
- ^ Internet Phones Call on Wi-Fi. PCWorld. Retrieved on 2006-05-26.
- ^ WiFi phones for Skype and SIP: available now, but be careful what you buy. Voipally. Retrieved on 2006-08-22.
- ^ Dual-mode cellular/WiFi handset adoption. TMCnet. Retrieved on 2006-05-26.
- ^ VoIP Security: Uncovered. WhiteDust security. Retrieved on 2006-05-26.
- ^ Global VoIP Policy Status Matrix. Global IP Alliance. Retrieved on 2006-11-23.
- VoIP at the Open Directory Project
Categories: Articles with unsourced statements since February 2007 | All articles with unsourced statements | Pages needing expert attention | Articles lacking sources from June 2006 | All articles lacking sources | Telephony | VoIP terminology & concepts | Broadband | Voice over IP