Doorking is one of the three main enterphone companies that is seen in North America today. A fun feature of these apartment call boxes is that they have a common service key to allow for maintenance of the circuits inside. Most of these boxes also have a place to put a postal lock to allow the mailman into your building to deliver the mail. If you are able to use the key to open the box and then jumper-out the two terminals that would normally connect to the postal lock (even if one isn’t installed), this will cause the door to open.