As against many radio technologies, telephony generally works in the full-duplex system and allows for the intercom function. Depending on the underlying technology, various methods are used to achieve the exchange of information in both directions via a common medium. Analogue telephony uses the so-called hybrid termination circuit. It ensures that the two-way voice signals are able to merge and separate. Generally in digital telephony, various duplex systems such as time division duplex, frequency division duplex or code duplex are used to separate voice directions e.g. through separate frequency ranges or transmission time slots.
Mobile phone networks mostly use a combination of frequency division and time division multiplex. The user sends transmission data via his/her mobile phone via a special uplink frequency and receives data via the downlink frequency in a specific time slot.
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.