An intercom (intercommunication device), talkback or door phones is a stand-alone electronic communications system intended for limited or private dialogue. Intercoms can be portable but are generally mounted permanently in buildings and vehicles. Intercoms can incorporate connections to warlike talkies, telephones, cell phones and to other intercom systems over phone or data lines and switch electronic or electro-mechanical devices such as signal lights and door latches. Intercoms are not to be confused with "Telephone Door Entry" systems, where the 'Intercom' facility is an option available on a system in which instruments within a building can be used to intercommunicate with each other (usually a single-dwelling), or where apartments can communicate internally with a resident porter.