Asterisk

With the development of IP-telephony systems, it became possible to implement nearly all of the logic PBX (PBX) programmatically.

Imagine that you have three simple SIP-phone, and is to make possible communication between the three subscribers. Of course, you can prescribe in each phone IP-address of the other two, comparing them to short code dialing (most SIP-devices allows). But the very first inconvenience will come out when there will be 4-th SIP-phone - you have to reconfigure all existing phones.

SMS Solutions Now suppose that you have a server that is connected independently of each SIP-phones. And the whole logic of calls handled it: SIP-phone only sends him dialed number and establishes two-way voice connection. It turns out a kind of IP-analog ordinary PBX Iphone VoIP. Now imagine that the server can communicate with other similar servers, supports PCI-board analog and digital telephony (to combine with traditional telephone networks), integrated with the database and various external processors, and supports interactive menus, voicemail, all calls and many other extensions of functionality, such as speech synthesis and recognition, reception and transmission of faxes. So this is Asterisk.