Asterisk® – Open Source Telephone System
http://www.asterisk.org/

Asterisk® is the most popular and extensible open source telephone system in the world, offering flexibility, functionality and features not available in advanced, high-end (high-cost) proprietary business systems. Asterisk® is a complete IP PBX (private branch exchange) for businesses, and can be downloaded for free. This has been added to World Wide Web Reference Subject Tracer™ Information Blog.