as in reinventing the.

There are several inexpensive, reasonably fast, small, dtmf receivers out
there.  A 14pin DIP package I've used recently with great success, as a
peripheral to a 16c84, is the motrola mc145436 dtmf decoder.  It does the
filtering, detecting, and decodiing for you.  It is very friendly to
interface, has a large (accurate) dynamic range, and rarely if ever
falses.

Although it would be an accomplishment, doing this in software on a pic
(no dsp engine) will be difficult, time-consuming and resource eating.

I'm all for making pics do as much as possible, but you'll have a tough
time making a cost-effective replacement for a <$2 part like this.

I've also had success with SSI202 and M8880 varieties.  For the SSI202
and MC145436, you just monitor strobe and four bits of data... the M8880
is a transceiver and requires a bit more manipulation.

Best regards to all,
Mike