OSIP is an LGPL implementation of SIP.
It's stable, portable, flexible and compliant! -may be more-!
It is used mostly with eXosip2 stack (GPL) which provides
simpler API for User-Agent implementation.

SIP stands for the Session Initiation Protocol and is described
by the rfc3261 (wich deprecates rfc2543).
This library aims to provide multimedia and telecom software
developers an easy and powerful interface to initiate and
control SIP based sessions in their applications.
SIP is a open standard replacement from IETF for H323.