you can find DTMF decoders and associated gear at any ham radio dealer
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the tones (called dtmf tones) are a combination of 2 sounds each(on the phone try 123 vs 147 to see the difference)they can be decoded with ffilters for the appropriate frequencies (you can find online) or with prebuilt dtmf chip like here
. Yep. Searching for DTMF decoder turns up a lot of good stuff.