Jacob Reider directed me to this one:
Asterisk is a complete PBX in software. It runs on Linux and provides all of the features you would expect from a PBX and more. Asterisk does voice over IP in three protocols, and can interoperate with almost all standards-based telephony equipment using relatively inexpensive hardware.He's put a home PBX in his basement. This is one of the more astounding Linux developments I've come across; it feels like the start of something quite interesting. I wonder if we'll see an array of low cost embedded Linux capabilities of this sort.
Asterisk provides Voicemail services with Directory, Call Conferencing, Interactive Voice Response, Call Queuing. It has support for three-way calling, caller ID services, ADSI, SIP and H.323 (as both client and gateway). Check the Features section for a more complete list.
No wonder the phone companies are working hard to destroy VOIP telephony.