If you live in the residential building equipped with intercom, yo can add some kind of intelligence to the unit in your flat. Just detach the ringer (buzzer, beeper, whatever), conduct the signal to the microcontroller in some convenient way (optocoupler or relay) and let it ring and open the door lock under the firmware control.

So if you don't have the key, you can type your "secret Morse password" (say, short-long-short-short) on the button at the entrance, and open the door without disturbing anyone. If the password is not recognized, normal ringer signal will be generated - or nothing will happen, depended on the state of S1 - just switch it on if you don't want to be disturbed by someone who does't know the password.