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 [http://asterisk.org/index.php?menu=support].  Newcomers with questions are encouraged to 
search the mailing list archives or wiki before posting, as list traffic is fairly high.
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More traditionally, ''asterisk'' is also the name for the character "'''*'''", which is sometimes 
pronounced as ''splat''.