Asterisk is an open source software that transforms a computer into a private or web-based telephone exchange.  Asterisk is issued with a  GPL license.

Asterisk enables voicemail, conference calls, voice servers, and a multiple destination calls amongst others. Asterisk uses protocoles H.323 and SIP protocols as well as a specific protocol named which allows communication between Asterisk servers as well as between the client server and the Asterisk server.

Asterisk can also connect to public networks (RTC, GSM etc.)

Asterisk : lowers your costs

When combined to a low-cost telephone service provider, Asterisk can be used as a low-cost PBX while also offering even more functionalities than usually included in the highest priced telephone systems.

Asterisk : controlling your phone system 

Asterisk gives you the power to directly control the functioning of your phone system. Once a phone message enters a Linux server empowered with Asterisk, virtually anything can be done with that message. This is due to the flexibility offered by open source technology. 

Asterisk : development and implementation

Asterisk permits the rapid customization and deployment of applications for your IP-PBX. The power of its CLI and configuration files delivers real-time diagnostics and installation capability. 

Asterisk : its many beneficial facets

As an open source software, Asterisk gives you with numerous options that empower you to shape your system as you want it e.g. picking up phone calls by email, setting up audio IVR menus, and voice conferencing. These are usually very expensive features that involve proprietary software. In addition, numerous new functionalities can be installed with minimum effort as they become available.

Asterisk : Customization 

A good part of the Asterisk popularity is due to the fact that its configuration files, source code and each functionality can be customized. This means that the Asterisk source code can be changed to fit the source code of your existing telephone system.

Asterisk : Dynamic content

Just like the capability of Apache servers lets you create dynamic content on the internet, so too ASTERISK lets you create dynamic content for your phone system....and with the greatest of ease.

Asterisk : an impressive flexibility

The majority of existing  Asterisk functionalities are truly easy to activate...and future ones are are to be designed similarly. Last but not least, Asterisk has an extraordinary number of tested extensions that are ready to install.