a software based private branch exchange (PBX)
Asterisk was originally developed by Mark Spencer in 1999. As a PBX, it allows phones and other telephony devices to call each other. Asterisk also provides the ability to integrate multiple telephone services like VoIP and the public switched telephone network.
Asterisk provides all the functionality expected in a PBX, such as voice-mail, interactive voice recordings (IVR), call forwarding, and conferencing.