Decoding DTMF in software is harder. Here is one example in asm: http://www.instructables.com/id/DTMF...-Microp/#step1

And here is Microchip's app note AN257 http://www.microchip.com/stellent/id...pnote=en024294

00257a.pdf

Probably easier to stick with your 8870, or something similar.