OpenSIPS (Open SIP Server) is a mature Open Source implementation
of a SIP server.

It includes application-level functionalities and is the core component
of any SIP-based VoIP solution. With a very flexible and customizable
routing engine, OpenSIPS unifies voice, video, IM and presence
services in a highly efficient way, thanks to its scalable (modular)
design.

It is reliable and fast, for enterprise or carrier-grade use cases.

WWW: http://www.opensips.org/