PJSIP and PJMEDIA are open source, high performance, small footprint SIP and media stack written in C language for building embedded/non-embedded VoIP applications.
This is where we talk about pjsip development in general, and maybe other stuff as well. For the project itself, please go to the main pjsip project website.
PJSIP is backed by Teluu, which provides professional support, additional licensing options, and a network of qualified consultants.
pjsip blog 