A door intercom is a system which enables you to remotely check who is at the door. The system is often used in flats and apartment complexes, where due to the size of the building the residents cannot see who is ringing the doorbell downstairs. It is also used when a house or apartment complex is equipped with an entrance gate.
A two-way communication system is usually used. After the doorbell rings, the resident can speak to the person standing at the door or gate. If the resident wishes to grant the visitor access, he or she can press a button to open the door or gate remotely. Modern systems are often equipped with a video camera so the resident can see who is at the door.