Asterisk

Open Source PBX Phone System http://www.asterisk.org https://en.wikipedia.org/wiki/Asterisk_(PBX)

User for VoIP and POTS.

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