Teltone produce a range of DTMF decoders that give the 4 data bits and a
"Valid Transmission output". You simply poll VT till it goes low (or hook it
into interrupt pin), then read the 4 bits for that digit. Just one thing to
watch, "0" is read as "1010" (10).

The chip is MV8870. The site is http://www.teltone.com

Regards
David Huisman
http://www.orbitcoms.com