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.
More traditionally, "asterisk" is also the name for the character "*".