It is often possible to integrate the functions of intercom devices, as found on apartment or house doors, into a telephone system. In such a case, the call participants can communicate with visitors at the house door via the telephone system using the intercom function. When a visitor rings the door bell, it is signalled in the same way as a telephone call and the intercom function allows communication with the visitor like a normal phone call. If mobile telephones are integrated into the telephone system, then the intercom function with visitors can, in principle, be used at any desired location. Mostly, it is also possible to open the door directly via the telephone by pressing certain function keys.