When I lived at my previous apartment, I had a problem with receiving packages. This would have been frowned upon by my landlord, but I used a combination of LittleBits (Arduino unit, 2 servo motors, and a light sensor), rubber bands, tape, Sugro, and Instamorph to build a system that opened the door for anyone who buzzed my unit.
The light triggers the light sensor, which lifts the servo motor on the phone hook. After a couple seconds, another servo button hits the door open button. A few more seconds, and the first servo motor depresses the phone hook to hang up. When it worked, this system would admit anyone who dialed my unit…