YATE is a telephony engine aimed at creating a telephony server that performs well enough to deal with PBX requirements and also flexible enough for complex Gateway and IVR solutions.

YateClient is an Instant Messenger and a SoftPhone.

Supported Instant Messaging Providers
Google talk/Google Voice
Facebook chat
jabber.org
Supported Telephony Protocols
Jingle/ Google Talk/ XMPP
Session Initiation Protocol (SIP)
Packet-based multimedia communications systems (H.323)
Inter-Asterisk eXchange protocol (IAX)