An IP PBX or VoIP phone system replaces traditional PBX or phone systems, giving employees an extension number, the ability to conference, transfer and dial other colleagues. All calls are sent via data packets over a data network instead of the traditional phone network. With the use of a VoIP gateway, you can connect existing phone lines to the IP PBX and make and receive phone calls via a regular PSTN line. The IP PBX FAQ helps answer common questions about VoIP, SIP, IP PBX / VoIP phone system hardware and software, implementation and more.
This list shows some of the currently available SIP-based IP PBXs in the telco sector:
A number of open source alternatives are available, but these all suffer from: