Asterisk

AsteriskTM is an open source PBX system that includes all the features of high-end PBX systems at a fraction of the cost. AsteriskTM can provide interactive voice response, conference calling, integration of computer and telephone systems, and can send voicemail messages to e-mail.

Asterisk reports up-to-the-minute caller activity, provides seamless home, mobile, and remote office extensions, and enables you to save money on your telephone bills.

http://www.digium.com