You can use jingle framework.This is what jingle wiki says:
"Jingle is an extension to the Extensible Messaging and Presence Protocol (XMPP) which adds peer-to-peer (P2P) session control (signaling) for multimedia interactions such as in Voice over IP (VoIP) or videoconferencing communications. It was designed by Google and the XMPP Standards Foundation. The multimedia streams are delivered using the Real-time Transport Protocol (RTP). If needed, NAT traversal is assisted using Interactive Connectivity Establishment (ICE)."
The libjingle library, used by Google Talk to implement Jingle, has been released to the public under a BSD license. It implements both the current standard protocol and the older, pre-standard version
Also there is a framework in Google project repository.
http://code.google.com/p/libjingle/
Also, one more link related to this:
iOS: Open Source VoIP/SIP Objective-C Code
May be this will give you an idea.
