pjsip.org
http://www.pjsip.org [1]
http://www.pjsip.org [2]
Windows, Windows Mobile, Linux, uC-Linux, Unices, MacOS X, RTEMS, SymbianOS, etc.
An Open Source SIP and media stack with good features [3], portability, performance [4], and small footprint [5], written in C.