A typical intercom has an amplifier, two or more stations each with a microphone and speaker or a two-way (microphone or speaker) transducer and a switch interconnecting the microphone, speaker and transducer with the amplifier for communication between the two stations. Generally, these systems connect the stations to each other through individual audio links, the number of which is far less than the number of stations in the system. A call is initiated when a calling station sequentially scans the audio links in search of an unused link and, when it finds an available link, connects itself to that link. The calling station then generates the address of the called station by dialing with either a dualtone multi-frequency device or a rotary dialing device. Link controllers associated with individual audio links decode the address of the called station and cause the called station to connect itself to the audio link to which the calling station is connected. The conference-type intercom systems include a switch matrix, commonly referred to as a crosspoint switch, which allows any user to communicate with any selectable mix of the remainder of the users. With crosspoint switches, all stations are connected directly to the crosspoint switch matrix which makes connections between the sources and destinations internally. Crosspoint switches are normally used instead of direct point-to-point connections between source and destination equipment for all but the smallest implementations. In order to accommodate large numbers of switched audio channels, intercom systems typically distribute the crosspoint matrix across a number of bussed circuit modules or cards. Wired intercoms for facilitating communications between crew members of military vehicles such as tanks and armored personnel carriers are well known. Such contemporary wired intercom systems comprise a wired communications system within the vehicle itself, as well as headsets which must be worn by the crew members and plugged into the wired intercom system of the vehicle. A wireless intercom requires the pressing of an emission button in order to emit signals. An external earphone type wireless intercom also requires the pressing of an emission button in order to emit signals, and the user has to speak through a microphone.

The use of an intercom system in a residence is well known, with intercoms often being provided at the front door of the residence so that the inhabitants of the residence can converse with a visitor standing outside at the front door. A modern home potentially has a great number of sophisticated electrical systems, including security systems, audio/video systems, telephone systems, intercom systems, etc. Typically, an apartment building or an office building may have a single entrance leading to several apartments or offices, such as through one or more hallways, stairways and/or elevators. Usually, there is a locked door at this entrance and a wired intercom system enabling a visitor to call any selected apartment or office from the entrance and have voice communication back and forth with the person who answers the call in the apartment or office. An intercom system for a house comprises a doorside intercom set and an indoor intercom set. The intercom includes a microphone for receiving verbal communications from the visitor standing at the front door, and includes a speaker for audibly relaying to the visitor comments made by the inhabitants of the residence. With the intercom system, a visitor at the door can use the doorside intercom set to exchange with the host using the indoor intercom set. When the visitor presses the push button of the doorside intercom set, the indoor intercom set will ring. The intercom system also has wiring which enables the person in the apartment which has been called to release the door lock at the entrance to give the visitor access. In some door intercoms, the speaker and microphone are wired to a digital signal processor. The digital processor is spliced into the wiring leading from the doorbell button to the doorbell of the residence. The digital processor is also connected to a public switched telephone network (PSTN) just as if the digital processor were an ordinary telephone. When a visitor pushes the button of the doorside intercom, the telephone as well as the indoor intercom set will ring. This function enables the host to use any of the extension which is nearest to him to identify the visitor and the visitor's intention, without going to the indoor intercom set which may be a distance from him.