Would putting the MP3 on your phone and playing it into the entrypads mic get you in as well? I'm picturing waling up to any apartment building, choosing an apartment number at random, and then just playing the MP3 when someone answers (or when voicemail picks up.)
I doubt it. It's listening for the 9 tone on the other end and I think it can distinguish between the two.
This reminds me of a hack we did in my freshman dorm though... we cut the wire from a card reader to a washing machine and recorded the signal the reader sent. Then reconnected the card reader to the washer in parallel with an 1/8inch audio jack. The folks who were in the know would plug their iPod into the audio jack on the back of the washer, play the "free laundry" mp3, and get a free load of laundry.
I'm sure some systems are vulnerable to this (although the mic might mangle the DTMF). After all until the late 80s the entire phone system was vulnerable to these kinds of attacks because it used in-band signaling
I was thinking of doing this with my answering machine, simply put the '9' MP3 at the beginning or end of my message so that if my wife isn't in and I forget/lose my keys that I can still get in the building.
Although it's a little redundant as I know so many people in the building that I'd probably be let in before the voicemail played the right tone. Plus my in-laws aren't a far walk for me to crash on their spare bed or pick up my spare set of keys.
However, I basically just want to do it to see if it works.
