Door_phone
A door phone or door bell phone is a set of electrical and electronic elements used to handle communication between a resident in a house, apartment or villa and a guest outside. The device can also lock or unlock the door it has been configured to work with. Door phones have been used across a variety of commercial and residential buildings. For example, offices and apartment blocks both make frequent use of door phones. They are so widely used that, nowadays, they form part of the standard electrical installation of most buildings.[1]
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The simplest version is an intercom that establishes a communication between an entryway at street level and a resident inside the house. A loudspeaker installed at the street level entrance allows a building resident to speak to their visitor. In buildings where there are more than one door phone plate located outside of a building's entrance, each door phone has a certain number of buttons depending on the number of units in each building.