Asterisk is a full featured Open Source (free as in GPL) PBX software for Linux (Asterisk also runs on FreeBSD, and possibly other platforms but does not support the Digium hardware on platforms other than Linux).
VoIP protocol support
Asterisk supports a wide range of VoIP protocols including SIP products such as the GrandStreamBudgetTone VoIP phones, Cisco VoIP phones and various others including softphones as well as H.323, IAX and MGCP.
Asterisk supports GSM, Mu-LAW, A-Law, ADPCM, iLBC as well as G.729 (available with a commercial license).
Traditional Telephony
In addition to supporting native VoIP networks, Asterisk can also talk to the traditional telephone network, supporting the following signal types:
- Robbed Bit Signaling Types
- FXS and FXO
- Loopstart
- Groundstart
- Kewlstart
E&M
E&M Wink
- Feature Group D
- PRI Protocols
- 4ESS
- Lucent 5E
- DMS100
- National ISDN2
- EuroISDN
- BRI (ISDN4Linux)
Asterisk is fully extensible both through a C level API and the AGI (Asterisk Gateway Interface} allowing custom call applications to be developed in any programming language of course.
Asterisk ships with modules for Voicemail, forwarding, call queuing, Enum164 and many other applications.
Suggested reading
The voip-info Wiki
The complete summary of built-in Asterisk applications and functions
- #asterisk on irc.freenode.net
