Physically Accessing Your Apartment with Skype

by dopamine (Aubrey Ellen Shomo)

I live in one of those apartment buildings that has a callbox for entry.  You know, one of those systems with a tenant directory that calls the tenant and allows them to let you in by pressing a key on their phone.  My box has no code for entry so the metallic key is the only approved way to get in.

I also misplace my keys quite a bit.  So I had this great idea.  Why not have my voicemail message buzz me in?  Thanks to that simple idea, I learned how difficult it is to find a voicemail system that will actually record DTMF tones.

Almost all voicemail services are DTMF controlled and stop recording on a tone.  I have access to a couple of different VoIP services that will email voice messages, but won't let me upload a WAV file for my greeting.  Even store-purchasable answering machines tend to not let you put in DTMF.

I figured I had three options.  I could write a program to answer a call and send the correct tone using a modem or SIP.  I could find another way to trick the door into opening (pink noise and the DTMF tone from the outside of the callbox, maybe?).  Or I could find a way to get DTMF into my voicemail message.

Luckily, I just got hooked up with SkypeIn.  Unlike other voicemail systems, this one lets me record a greeting from my computer.  Still no upload for WAVs, but at least they don't stop recording on DTMF.

I had another problem.  My area code has no SkypeIn numbers and I didn't think I could get my landlord to program a toll call into the box.  Solution: Call forwarding on busy/no answer.  Plus with my VoIP service, I don't pay long distance for the forward.

So with that, all there was to be done was to get the DTMF tone onto my voicemail greeting.  I tried just boxing it by holding a tone generator up to my mic for the first go-round.  No luck.  Computer microphones are pretty crappy these days.

The solution was a simple WAV editor.  Most sound cards can use their own wave (software) output as a record input, so I recorded the tone from a software DTMF generator within the sound card, then added on my regular message with a mic.  With a little editing, I had a nice message that sounds to a normal phone user like a tone followed by my voicemail greeting.

After creating the WAV file, just set your record input to your WAV out again, tell Skype to record a greeting, play the WAV file, then stop the record.  Presto.  You have a number you can call that will generate a predefined sequence of DTMF tones automatically without human intervention.

This trick would work just as well, of course, with a proximity card system that lets you buzz people in as long as you live there and can set the number it calls, or forward from that number.  And it's a lot easier to misplace (and not have duplicates of) a proximity card.

Of course, the same trick would let you get into any apartment building where you could access the copper for the phone lines.  Just punch in call forwarding to a SkypeIn account with the right greeting from one of the lines in the building and buzz yourself in.  You'd have to match a line with a name, but that's not too hard.  With forwarding and a DTMF-friendly greeting, you don't have to have someone standing there in the phone company box while you try to get in the door and you don't have to socially engineer anyone into just letting you in.

So it works at unusual times when more straightforward approaches would fail, or at least attract undue attention.