Asterisk - Open source PBX

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

http://www.asterisk.org/features