What is Asterisk?
Asterisk is an open-source software program published by Digium that you can use to enable a PC to run as a server for a VoIP service. Asterisk can also be used to develop telecommunications applications.
Because Asterisk is an open-source system, you have full access to Asterisk’s source code. With this open source software, you can develop your own services and features based on Asterisk's capabilities and components. Asterisk software is free to download and use. Generally, you shouldn't need any extra hardware with Asterisk either.
The biggest selling point for Asterisk is that it allows you to turn any computer into a fully functional IP PBX. Asterisk was made to run on Linux, but other Asterisk servers include BSD, Windows, and Mac OS X. However, with a bit of technical knowledge, you can configure Asterisk to run on pretty much any operating system.