The Main Gate and Doors with Keypads have unique numbers they use when someone calls you from there to get in. You can add these to your phone's contacts so you recognize the Caller ID instead of seeing some mystery number.
For example, when someone calls me from the front gate or a door with a keypad to let them in my phone shows:
Gate - Press 9 or Door - Press 9
(after speaking with your guest, you press "9" to let them enter)
Contact Name: Gate - Press 9
(make this contact a company)
Contact Number: (832) 230-1975
Contact Name: Door - Press 9
(make this contact a company)
Contact Number: (281) 741-5060
Voilá! Now when someone calls you from the Main Gate or a Door you will recognize the call instead of seeing a mystery number.
Try it yourself - take your phone to a door with a keypad and enter your apartment's code on the keypad - answer your phone and unlatch the door by pressing "9" on your phone.
Consider giving both numbers a distinctive ring-tone as well.