Sofia-SIP is an open-source SIP User-Agent library, compliant with the IETF
RFC3261 specification (see the feature table). It can be used as a building
block for SIP client software for uses such as VoIP, IM, and many other
real-time and person-to-person communication services. The primary target
platform for Sofia-SIP is GNU/Linux. Sofia-SIP is based on a SIP stack
developed at the Nokia Research Center. Sofia-SIP is licensed under the LGPL.

WWW: http://sofia-sip.sourceforge.net/