Asterisk http://asterisk.org is a telephony integration platform. It provides a complete PBX, interfaces for analog and digital hardware (T1, ISDN), VoIP (voice over IP) protocols, computer-telephony integration, IVR (interactive voice response) and ACD (automated call distribution) programming.
Asterisk can be programmed in Tcl using the agi package. A Tcl/C-based softphone is also available at the Tel project http://tel.sourceforge.net
As of June 2004, Asterisk is very stable, but still somewhat evolving.
A very extensive Asterisk wiki is avaliable at http://www.voip-info.org/wiki-Asterisk .
Mailing lists for users and developers, including an archive of past messages can be found at [L1 ]. Newcomers with questions are encouraged to search the mailing list archives or wiki before posting, as list traffic is fairly high.