Yate is a softswitch (concept of separating the network hardware from network software). It supports VoIP protocols: H.323, SIP, IAX1, IAX2, RTP. For the plain old telephony systems (PSTN) it supports ISDN BRI and PRI. Is compatibile with EuroISDN, National ISDN, Lucent 5E, Nortel DMS-100, NI-2. Yate can be used for:

VoIP (SIP, H323, IAX) - PSTN gateways PC2Phone prepaid or postpaid calling systems Calling card applications Call centers

The software is written in C++ and it supports scripting in various programming languages. YATE's license is GPL with an exception for linking with OpenH323 and PWlib (licensed under MPL).