One of the puzzles +Sai and I wrote for the 2011 Mystery Hunt was Making the Possible Impossible -- sadly, we didn't manage to keep the server running it was hosted on.  It was a phone-tree based puzzle which hinted you at various points that you needed to perform certain phreaking-style tasks.  Our Asterisk back-end supported detecting most of the phreaks out of the box, but one it didn't was the old coin tones, simultaneous 1700 Hz and 2200 Hz
So we quickly-and-dirtily hacked these up an extra pair of frequencies into the Asterisk DTMF tone detector for our puzzle, and being good open-sourcely citizens threw it up on github:

And it paid off!  +Jered Morgan and Teli Brown have used our patch in getting a dial plan functioning for an old payphone that produces the coin tones.  In Jered's words:
After struggling to find an ATA that would accept the Payphone's unually high REN(Ringer Equivalency Number), we got it working on an AudioCodes MP-202(which is designed for fax machines, hence why I guess it doesn't mind the high REN as fax machines tend to have higher RENs that regular phones).  In any event, we created a dial plan and an extension for the payphone, set all outbound calls to ask for 25 cents and it works like a charm!  The code does seem to miss a quarter tone about 20% of the time, which is not bad considering it was nearly 90% of the time in our testing with a redbox, I'm glad to see it working almost every time with the genuine payphone, though I suspect this is either a timing issue in the code that can be patched or the need for a wideband codec, further experimenting and time will tell.  One really awesome feature is, since Asterisk thinks the coin tones are DTMF, it filters the coin tones from reaching the called party thinking you pressed a DTMF button, but since the patch only detects and doesn't ENCODE coin tones, it in essence squelches the coin tones from the called party!  It's a FEATURE not a bug, because if this were a true Payphone voip deployment, and someone was trying to call their buddy or an answer machine to pop some coins in to record the tones, the far end can't record the tones! :)

You can try a proof of concept at 209-644-ACTS; press * after the tone to go to the coin test.

Main link is Phil Lapsley's blog where something on this might be appearing soon.
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