The apartment building I live in uses a system where visitors can type in an apartment’s number on a keypad in the lobby, which calls the phone in the apartment. Someone in the apartment would then pick up the phone, confirm that the visitor is not Charles Manson, John Wayne Gacy, or Boba Fett, and then dial the number 9. The door of the building would then unlock, and the visitor could proceed up to the apartment, perhaps for a tea party.
The problem with this is that my roommate and I were too lazy to pick up the phone every time someone wanted in. My roommate, a fellow engineer, had concocted a plan to build a circuit that would answer the phone call and then generate the DTMF tone for the number 9, the pair of 1477 hz and 852 hz frequencies. The obvious flaw in this was that if we were too lazy to walk 5 feet to the phone, there was no way we were going to build and debug this circuit. I had then planned to use my Asterisk box to pick up the call and generate the tone, but the prices on decent FXO cards were way out of my budget.
No, my friends, the solution was not a carefully crafted circuit or a complex piece of software but a lowly answering machine. I used Matlab to create a WAV of 1477 hz and 852 hz sine waves. We hit the “record answering machine message” button, cranked up the speakers, and blasted the number 9. Now, when someone buzzes in, the phone silently rings for a moment, the answering machine plays the 9 tone, and like magic, the door unlocks.
It may not be the most elegant solution, but its simple, robust, cheap, and easy. It’s been working for the last year and a half, and many of the other people living in the building are using it too. Of course, it completely eliminates security, but if you wait in the lobby for a few minutes and don’t look like Jack the Ripper, some kind soul will let you in anyway.