Some telephones, depending on the brand and model, have the intercom function, which allows to begin communication by sending the signal. When the signal comes, the telephone set rings a signal different from the standard ring tone call. After that the telephone temporarily switches off from the telephone line and works as an intercom.
Citel PBXgateways are installed at the central PBX location, and EXTenders are deployed at branch offices, call centers, or at home offices over managed IP connections, DSL, cable, or analog circuits