Open source IP PBX heritage

All of the systems examined in this Clear Choice were based on Asterisk. From its inception in 1999, when Digium's Mark Spencer developed the Asterisk IP PBX, it has been one of the most widely used and freely downloaded open source GNU General Public License (GPL) products in the VOIP telephony market.

Asterisk architecture is based on the Inter-Asterisk eXchange (IAX) protocol and supports Session Initiated Protocol (SIP). Digium also created an online forum for developers in which they can collaborate by submitting bug-fix reports and providing feature code for review that could be included in a subsequent release. Digium makes the basic Asterisk code available to everyone and earns revenue by offering several levels of technical support and selling peripherals such as FXO/FXS and T1/E1 PCI cards.

Another vendor, Pingtel, announced in 2004 that it was moving its previously exclusive SIP SIPxchange IP PBX software into the open source realm. It created an online organization called SIPfoundry with forums to share and review additions to the core code much like Asterisk and include them in the next release. This approach also allows Pingtel to adapt the open source code into solidly distributed retails versions as the code matures.

Pingtel is regarded in some cases as the SIP industry's leader because of its many Internet Engineering Task Force (IETF) SIP contributions and the development of its IP PBX code The Pingtel business model is similar to that offered by Red Hat with its Red Hat Linux line of products and service and support organization. It is based on offering free code with the option of obtaining stable retail versions and requiring annual subscriptions for technical support and new releases.