An intercom for a building entrance: what the system consists of

An entrance intercom is not one device but a system on the door and in every flat. These are its parts.

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Structure

Six parts

Call panelIt is mounted by the door. The flat number is dialled on it; in a video intercom it has a camera.
LockUsually electromagnetic: it holds the door while there is power and opens on a signal.
Exit button and door closerFrom inside the door is opened with a button, and the closer shuts it behind everyone who enters.
KeysResidents open the door with an electronic key or fob. In many systems a lost key can be removed from memory so that it no longer opens the door.
Units in the flatsA handset or a monitor for answering the visitor and opening the door.
Calls to a phoneAn IP intercom can forward the call to an app: the resident answers and opens the door even when not at home.

Questions

Which is better — handsets or calls to a phone?

A handset is simpler and more familiar. Calls to a phone suit those who are often out. The two can be combined.

Can a camera be added at the entrance?

Yes. A camera at the entrance and in the yard is installed together with the intercom or later.

Do you connect building entrances?

Yes: Hikvision audio and video intercoms, call panels, connection of flats, offices and building entrances.