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IP Telephony is the transport of phone calls through the Internet. IP Telephony services are available between computers and telephones. PC-to-PC is suitable for users who already have Internet access and an audio-equipped computer. Software is available for free. Integration with Internet services such as instant messaging and e-mail are provided. PC to telephone uses a gateway for call conversion from digital to analog and analog to digital.

Telephone-to-telephone requires two gateways, GSTN-to-Internet, and Internet-to-GSTN. IP Telephony services are either network-centric or end-system-centric. Mobility features are included in IP Telephony. IP Telephony services use either VoIP or SIP based services. VoIP systems are more popular, and their applications are generally used with a simple microphone and computer speakers. However, IP telephones or VoIP boxes can also be used. In recent years, the quality and reliability of VoIP technology has improved to a great extent, and a vast number of users are opting for VoIP based telephone systems.

IP telephony applications require firewalls to pass their Real Time Transport Protocol (RTP) streams. SIP technology has a better mechanism to tackle this problem. SIP server can act as a registrar, proxy or redirect server. It features an application-server interface, presence support, SMS gateway, simplified accounting and authorization and server status monitoring. Linux based SIP software supports numerous codes and phonebook.

The traditional telecommunication operators have been providing special call services using their existing infrastructure that has been accomplished with Intelligent networking architecture. In IP Telephony, router congestion is the main hindering factor, because the audio quality is not on par with PSTN services. Quality wise, IP Telephony services are yet to match their PSTN counterparts.