Steve's right. Sampling the mixtures of dual frequencies in DTMF is not a good micro application. Its quite easy to synthesize DTMF tones if you make sure that your frequency table lengths are all relatively prime to each other... but that's another topic.

You need a DTMF decoder chip.

By the way, we looked at doing what you're attempting a Tandy's design lab about 15 years ago and decided the DTMF chip was worth the cost. For those that know their policy for incremental component costing, heheehe, that means it must have been almost impossible to save the price of the DTMF decoder chip.

-Jay C. Box