Asterisk — Open source PBX

Asterisk-based telephony solutions offer a rich and flexible feature set. Asterisk offers both classical PBX functionality and advanced features, and interoperates with traditional standards-based telephony systems and Voice over IP systems. Asterisk offers the features one would expect of a large proprietary PBX system such as Voicemail,Conference Bridging, Call Queuing, and Call Detail Records.

http://www.asterisk.org/features