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An error-correcting code would be even better (no more "*N* as in *Nancy*") but might be too long. Also, entering via dialtones is crucial, because you *really* don't want to risk having transcription errors in the checksum too.
Actually, adoption might not be too hard: Businesses could start offering support for entering a checksum *if you have one*, and otherwise confirming the usual way. Of course, in a few years there will be no more humans speaking over the phone anyway, just Google Duplex AIs :P
You'd need a unique canonicalization of the data format. Spaces? Punctuation? St. vs Street? 3rd vs Third? How about a mobile app that sends and receives modem tones instead?
Ha, that would really be something! Press a button to send your name/address/email at 300 baud 
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