Mobile VoIP !!!!

Mobile VoIP or 'Mobile Voice over Internet Protocol' is the application of voice over IP technology to mobile handsets.

Mobile VoIP requires a mobile handset that supports, at minimum, high speed IP communications. Most commonly this is using Voice over WiFi or VoWiFi, but the same protocols (typically SIP or jabber) can be used over any broadband IP-capable wireless network connection such as the various 3G standards, EVDO rev A (which is synchronously high speed - both high speed up and down), HSDPA or potentially WiMax.

Mobile VoIP will require a compromise between economy and mobility. For example, Voice over WiFi offers free service but is only available within the coverage area of the WiFi Access Point. High speed services from mobile operators using EVDO rev A or HSPDA with probably have better audio quality and capabilities for metropolitan-wide coverage including fast handoffs from mobile base station to another, yet it will cost more than the typical WiFi-based VoIP service.

Technologies

There are three main technologies used for mobile VoIP:

UMA - The Unlicensed Mobile Access Generic Access Network, designed to allow VoIP to run over the GSM cellular backbone SIP - The standard used by most VoIP services, and now being implemented on mobile handsets Skype Protocol - a proprietary modified version of Jabber

voip drawbacks.

Difficulty with sending faxes
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One drawback is the difficulty in sending faxes due to software and networking restraints 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 a 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
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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.[citation needed] 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.

VOIP !!!!!!

Voice over IP
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You can make all of your phone calls with your Internet broadband connection as opposed to an analog phone line (regular phone line). Voice Over Internet Protocol (VoIP), commonly known as Internet Voice, is the technology that makes this possible. There are some VoIP services that limit you to making calls only to people with the same service, but there are others that give you the freedom to call local, long-distance, international, and mobile numbers. Similarly, there are VoIP services that require you to make phone calls on a special VoIP phone or your computer, but others will make it possible to use a regular telephone through an adaptor.