Best apartment security system?

Seinfeld (TV series): When anyone comes to Jerry's apartment, people inside hear a buzz and they just say come on up ? What is the security system ? Is someone sitting in the entrance of the apartment blocking them until they hear from someone inside?

• Answer:

  He hits a button that unlocks the front the door. Most intercoms in systems like this have 3 buttons. You press the first one to talk, the second one to listen, and the third one to unlock the door.