Hacking your apartment's electronic entry system 15 years ago
An apartment I lived in had a similar system. I noticed that the caller ID showed what appeared to be a valid number. So one day I tried calling it while at the door... when it started ringing, I started hitting buttons on the keypad (on the door) and when I hit the # key (or maybe it was *), it picked up. I was then able to dial 9 from my phone, and it let me in.
I no longer live there, and don't have access to an apartment that has one. I'd be interested to see if that works on other systems. If anyone wants to try, let me know if it does.