Door phones are designed to connect to PBX and be programmed via any extension phones.
Here is the common application:
The person at the door would press the CALL button on the door phone making the receptionist extension ring. Then she/he can talk to the person and simply press a feature code to open the door. In addition to that, we can allow anyone in the office to use their phone and open the door (in case the receptionist is not there)
There are different door phones available in the market, analog, IP-based, IP video door phone and so on.