Asterisk is a widely-used open source telephony server. From VoIP gateways to conference servers, Asterisk is effective for building communications solutions of all kinds. Asterisk was a pioneer in the world of open source telephony. While we are reluctant to use cliches, it was truly disruptive. It's not uncommon to reminisce with open source telephony geeks about how magical it was to turn old Pentium II's into fully functioning and hackable phone systems. It was clear that change was in the air.
Originally developed by Mark Spencer to fill a personal need, Asterisk quickly grew to be Digium's (a company Spencer later founded) core technology.
While Lintel generally prefers FreeSWITCH (its MPL license is far more business friendly than the GPL; its easier to scale; and its more dynamically configurable), Asterisk remains an important and formidable player in the open source telephony arena.