To set this up: contact Westminster Housing and provide them with a telephone number, usually your landline number. The office will give this to the telecoms firm who will set up the connection. This will cost you £30-£35. The door release system doesn't always work with modern, digital phones (DECT/DCET standard). And, frankly, the sound quality is poor, particularly as you receive the call in your flat.
Visitors: by pressing the number of your flat, plus enter, on the entry phone at the block entrance door, your visitor will hear a message telling them which flat they have phoned and hear the phone ringing. (There is also a clear button to enable them to start again)
In your flat: once the visitor presses the button your designated phone will ring. You talk to your visitor at the main door: if they are new to the block you might explain that you will release the door, what floor you are on and which lift they will need. Then press the number ‘3’ on your phone and the main door will open for your visitor. (If you are already on the phone your visitor will hear an engaged tone and have to wait …)
Beware: strangers may press your entry phone number so unless you know the caller, please do not release the front door.
See also the notes on Access to the Block and Security.