Asterisk

Open Source PBX Phone System

User for VoIP and POTS.

http://www.asterisk.org

originally developed by Digium http://www.digium.com/

Jul'2003 http://www.onlamp.com/pub/a/onlamp/2003/07/03/asterisk.html |article Aug'2005 http://www.oreillynet.com/pub/wlg/7495 |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