Intercom/VoIP

An intercom (intercommunication device), 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 walkie talkies, telephones, and to other intercom systems.  Intercom systems, which allow for communication between spaces that are separated by distance or walls, are very common especially in large office buildings. Because they allow for ease of communication and therefore increase both productivity and security

Some VoIP (Voice over Internet Protocole) systems offer Intercom option to permit office discuss between them