Asterisk is a complete PBX in software. It runs on Linux and other Unix based systems and providesall of the features you would expect from a PBX and more. Asterisk doesvoice over IP in three protocols, and can interoperate with almost allstandards-based telephony equipment using relatively inexpensivehardware. Asterisk provides Voicemail services with Directory, Call Conferencing,Interactive Voice Response, Call Queuing. It has support for three-waycalling, caller ID services, ADSI, SIP and H.323 (as both client andgateway). Check the Features section for a more complete list. Asterisk needs no additional hardware for Voice over IP. Forinterconnection with digital and analog telephony equipment, Asterisksupports a number of hardware devices, most notably all of the hardwaremanufactured by Asterisk's sponsors, Digium. Digium has single and quadspan T1 and E1 interfaces for interconnection to PRI lines and channelbanks as well as a single port FXO card and a one to four-port modularFXS and FXO card.