"Asterisk is a phone system in software. It can replace large
and expensive business phone systems, powering thousands of
extensions, or it can help home users save money on long distance.
Because it's implemented in software, it is extremely versatile,
easy to customize, and easy to extend.
"You're probably familiar with the concept of a Private Branch
Exchange (PBX) already, though you might not know it. Almost every
medium or large business has one. A PBX is, basically, a simplified
version of a telephone company's exchange. Most PBXs provide
features such as direct touch-tone dialing of extensions, routing
of calls onto the public telephone network, and voice mail
services. If you're accustomed to dialing 9 to place an outside
call, or dialing a simple 3- or 4-digit number to reach another
extension, you're using a PBX..."