Asterisk: Give a Try

Asterisk is an open-source project sponsored by Digium. The primary maintainer is Mark Spencer, but numerous patches have been contributed from the community. As of this writing, it runs only on Linux for Intel, although there was some success in the past with Linux PPC, and an effort is underway to port Asterisk to *BSD. Digium also sells various hardware components that operate with Asterisk (see Resources). These components are PCI cards that connect standard analog phone lines to your computer. Asterisk has its own VoIP protocol, called IAX, but it also supports SIP and H.323. This leads us to one of Asterisk's most powerful features: its ability to connect different technologies within the same feature-rich environment. For example, you could have IAX, SIP, H.323 and a regular telephone line connecting through Asterisk


Asterisk provides Voicemail services with Directory, Call Conferencing, Interactive Voice Response, Call Queuing. It has support for three-way calling, caller ID services, ADSI, IAX, SIP, H.323 (as both client and gateway), MGCP (call manager only) and SCCP/Skinny. Check the Features section for a more complete list.

Resource:
www.asterisk.org
http://www.voip-info.org/wiki/index.php?page=Asterisk