Linuxwall: "SIP and RTP: Overview of a VoIP Communication"
Read this article, which discusses one of the most popular network applications: Voice over IP. There are quite a few industry protocols for Voice over IP; one of the most popular and widely accepted is IETF protocol SIP. As you read, pay special attention to the registering and calling process.
Click http://wiki.linuxwall.info/doku.php/en:ressources:dossiers:voip:sip_rtp?s%5B%5D=sip&s%5B%5D=rtp link to open resource.