eXosip is a GPL library that extend the capability of the oSIP library.
It aims to implement a simple high layer API to control the SIP for sessions
establishments and common extensions.  Once completed, this eXtended library
should provide an API for call management, messaging and presence features.

eXosip2 has support for:
 * registrations. (REGISTER)
 * call initiation and modification. (INVITE, re-INVITE)
 * other methods within calls (INFO, OPTIONS, UPDATE)
 * call transfer. (REFER)
 * reliability for provisional response. (PRACK)
 * sip event package. (SUBSCRIBE/NOTIFY)
 * event state publication. (PUBLISH)
 * instant messaging. (MESSAGE)

eXosip2 does not contain:
 * RTP.
 * audio interface
 * sdp negotiation.

This allow you to write any kind of SIP endpoint/gateway.

WWW: https://savannah.nongnu.org/projects/exosip