The Asterisk(R) Open Source PBX
http://www.asterisk.org/

Asterisk is a full-featured telephony server which provides Private Branch eXchange (PBX), Interactive Voice Response (IVR), Automated Call Distribution(ACD), Voice over IP (VoIP) gatewaying, Conferencing, and a plethora of other telephony applications to a broad range of telephony devices including packet voice (SIP, IAX2, MGCP, Skinny, H.323, Unistim) devices (both endpoints and proxies), as well as traditional TDM hardware including T1, E1, ISDN PRI, GR-303, RBS, Loopstart, Groundstart, ISDN BRI and many more.