It is also not a big problem to implement a DTMF detection algorithm on your 51 micro. Of course you need an AD converter (8 bit resolution is enough).

Search the internet for a Goertzel algorithm. It is an IIF filter of 2nd order that can reliably detect a single frequency.

So you will have to implement 8 different detection routines each for one frequency. Since Goertzel algorithm is recursive, you can handle with no more than 16 bytes of RAM. Only the detection time can not be faster than 100 ms and if you need high sensitivity, then you'll have to add an analog amplifier to rise the level of input signal or to use an AD of a better resolution.

Franc