Asterisk is an open source software PBX, created by Digium, Inc. and a continuously growing user and developer base. Digium invests in both developing the Asterisk source code and low cost telephony hardware that works with Asterisk. Asterisk runs on Linux and other Unix platforms with OR without hardware that connects your server to the traditional global telephony network, the PSTN.
"Short for Public Switched Telephone Network, which refers to the international telephone system based on copper wires carrying analog voice data. This is in contrast to newer telephone networks base on digital technologies, such as ISDN and FDDI. Telephone service carried by the PSTN is often called plain old telephone service (POTS)."
Asterisk gives you real-time connectivity on both PSTN and VoIP networks
With Asterisk as your telephony switching platform, PBX, you'll not only have a high-class PBX replacement. Asterisk is much more than the standard PBX. With Asterisk in your network, you can do telephony in new ways.
Connecting employees working from home to the office PBX over broadband connections
Connecting offices in various states over VoIP, Internet or a private IP network
Giving all employees voicemail, integrated with the Web and their E-mail
Building interactive voice applications, that connect to your ordering system or other inhouse applications
Giving access to the company PBX for business travellers, connecting over VPN from airport or hotel WLAN hotspots
...and much more
Asterisk includes many features only found in top-of-the-line unified messaging systems, like
Music-on-hold for customers waiting in queues, supporting streaming media as well as MP3 music
Call queues where call agents jointly handle answering incoming calls and monitor the queue
Text-to-speech system integration (the Festival Open Source and Cepstral Swift speech synthesis software can be integrated)
Call data record (CDR) generation for integration with billing systems
Voice recognition system integration (such as the Sphinx Open Source voice recognition software)
The ability to interface with normal telephone lines, ISDN basic rate and primary rate interfaces