Asterisk
Open Source PBX Phone System http://www.asterisk.org https://en.wikipedia.org/wiki/Asterisk_(PBX)
originally developed by Digium http://www.digium.com/
Jul'2003 article
Aug'2005 article
http://www.iaxtel.com/ Iaxtel.com allows Asterisk users and IAX clients to connect with each other over the Inter-Asterisk eXchange protocol.
List of US consultants (includes some in NYC).
Switch Vox has packaged it for increased usability http://www.switchvox.com/
So has Fonality
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