An IP PBX is an Internet-based variant of a telephone system ( PBX = Private Branch Exchange). The user of such an online telephone system can freely choose the connected telephone companies.
Essential service features (e.g. free telephoning between connected subscribers, call forwarding, switching, etc.) of a conventional telephone system are handled on a server rented, for example, in a data center. Conventional system technology is no longer required in households or offices. This variant of a telephone system is possible in combination with Internet-based telephony ( Voice-over-IP ). A SIP telephone or conventional telephone in conjunction with a special adapter for the SIP protocol is logically connected to the IP-PBX via the Internet. The switching functions take place on the server with the help of special software. There are many commercial vendors for this, but the open source software Asterisk , originally developed by Mark Spencer, is widely used. In addition to the usual dialing functions, this GPL software can also be used to implement answering machines, telephone conferences and complex voice menu controls.