In SX Microcontrollers, SX/B Compiler and SX-Key Tool, kjennejohn wrote:

Why, Paul, you never looked better. Been working out or something?

You need this FCC approved phone-interface device:
CircuitWerkes MPC2 DAA

The data sheet and ordering particulars are at:
http://www.broadcastboxe.com/mpc-2.htm

It costs $40 and they supply a label with the device that attaches to the outside of your product and saves you the jail time and heavy fines of violating part 68. It also provides didtal signals that tell you if the phone is ringing, and when the other party has hung up. Nice toy.

At this point, unless someone can tell you how to implement multi-pole filters to decode DTMF on an SX, I'd look for a DTMF receiver that can take those tones and turn them into some readily recognizable digital form. Perhaps put out 4 bit BCD. Then you can get someone here to explain how to take three digits (say, 2-5-5, as in your example) and make a single number out of them for display.
However, after an hours search, I couldn't find a quick source for these ICs. Velleman Kits used to have a kit that produced one-of-ten outputs for controlling relays, but I can't find it. H*ll, after searching several kit sites, I can't find anyone producing a kit like this.
Go to this Samsung data sheet site for one possible DTMF receiver chip, the KT3170:
http://www.secomtel.com/UpFilesPDF/PDF/Samsung/Telecommunication%20IC/Exchange/KT3170.PDF

And then there is a chip from Silabs.com, SSI202, a DIP-18 IC. I haven't looked for the data sheet.

Doing a Google search on "DTMF receiver IC data specs" found several hits. The first two pages consisted largely of projects dealing with ham radio repeaters and the like. Half the battle is going to be to find someone making these parts with decent data sheets; the other half is finding someone selling these parts. Maxim had no hits; DigiKey had several hits, but all were No-Stock; Mouser had Silab parts, but they were high-falutin' SMT 38-pin parts.

Good luck on this one,
kenjj

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