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 do you want to
be disturbed by someone who does't know the password.