oSIP is an implementation of SIP.

SIP stands for the Session Initiation Protocol and is described by the
rfc2543 (soon to be deprecated by latest revisions). 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.

WWW: http://www.gnu.org/software/osip/