Intercom systems

An intercom (intercommunication device), talkback or doorphone is a stand-alone voice communications system
for use within a building or small collection of buildings, functioning independently of the public telephone network. Intercoms are generally mounted permanently in buildings and vehicles. Intercoms can incorporate connections to public address loudspeaker systems, walkie talkies, telephones, and to other intercom systems. Some
intercom systems incorporate control of devices such as signal lights and door latches.