Asterisk is a complete PBX software Solution. It runs on Linux, BSD, Windows (emulated) and OS X and provides all of the features you would expect from a PBX and more. Asterisk does voice over IP in four protocols, and can interoperate with almost all standards-based telephony equipment using relatively inexpensive hardware.
Asterisk is the world's most popular open source telephony solution It has been in use since 1999 and is constantly evolving and improving and alarmingly the base product is free. Installing Asterisk on an ordinary computer turns it into a feature-rich voice communications server.
Asterisk makes it simple to create and deploy a wide range of telephony applications and services.
Code for Asterisk has been contributed from open source software engineers around the world. It supports a wide range of telephony protocols and includes rich support for the handling and transmission of voice over traditional telephony interfaces including analog lines, ISDN-BRI lines and digital T1/E1 trunks. Asterisk also features support for a wide range of VoIP protocols including SIP, IAX and H.323 among others. It supports U.S. and European standard signaling types used in business phone systems, allowing it to bridge between next-generation voice-data integrated networks and existing infrastructure.
Asterisk is released as open source under the GNU General Public License (GPL), and it is available for download free of charge. Asterisk® is the leading open source telephony project and the Asterisk community has been ranked as a key factor in the growth of VoIP.