Not a simple application but it can be done. Do a search on DTMF and you can see what is involved. It is much simpler if you use an external DTMF decoder IC but of course they do cost some and can be somewhat hard to find at times as most are out of production.
You sure he didn't just want to use a phone keypad seeing how he ask for it as in his post.? but if he want a cheap decoder all he needs to do is find a old phone the kind with the bell in it the heavy phone
don't have a you decoder you can use but the one's that are lite have a good keypad and decoder.
The PIC would need to use four data input pins to read the parallel binary number from the decoder IC and an additional input pin to read the (new) data ready strobe from the decoder chip.