Sofia SIP is an open-source SIP User-Agent library, compliant with

the IETF RFC3261 specification. 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.