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 somewhat evolving.