Nowadays the tendency is to use the voice over IP services. It is an amazing choice in the aspect of cost saving. The first there are cost savings by not building separate infrastructure for telephone network. The second the computers could be used as a telephone terminals and it is not required to buy telephones. And finally the costs for internet calls are convenient.

There are more VoIP (Voice over IP) public branche exchanges available for Linux which can serve VoIP services. These exchanges serve many interesting functions, i. e. call routing, call recording, voice menu (IVR), voice mail, call transfer, call listing, music on hold and many others. It is possible to connect the PBX to more providers and therefore save costs. It is possible to connect IP phone to the PBX not only from the local area network, but also from internet. The only requirement for the IP phone is the support of one of the protocols IAX2, SIP or H.323.