Yate - Yet Another Telephony Engine

is a next-generation telephony engine; while currently focused on
Voice over Internet Protocol (VoIP) and PSTN, its power lies in its
ability to be easily extended. Voice, video, data and instant
messenging can all be unified under Yate's flexible routing engine,
maximizing communications efficiency and minimizing infrastructure
costs for businesses.

Yate is production-ready software and is easily extensible.
Yate is licensed under the GPL.