Asterisk is a complete PBX in software.  It 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.

Asterisk provides Voicemail services with Directory, Call Conferencing,
Interactive Voice Response, and Call Queuing.  It has support for
three-way calling, caller ID services, ADSI, SIP and H.323 (as both
client and gateway). Check the webpage's Features section for a more
complete list.
