Asterisk is a fully Open Source, hybrid TDM and packet voice PBX and IVR platform.
Asterisk is and has
been Open Source under GNU GPL (with an exception permitted for linking with the
OpenH323 project, in order to provide H.323 support). Commercial licensing is available
from Linux Support Services, Inc. (http://www.linux-support.net) for applications
in which the GPL is inappropriate.
Unlike many modern "soft switches",
Asterisk can use both traditional TDM technology and packet voice (Voice
over IP and Voice over Frame Relay) protocols. Calls switched on TDM interfaces
provide lag-less TDM call quality,
while retaining interoperability with VoIP packetized protocols.