Ok. thanks.
I found a nice projects for decoding DTMF without kt3170. If some friends interest it, The project here: **broken link removed**
This circuit uses PIC's comparator. For detecting DTMF tone.
8870 is dedicated to DTMF detection and has all the appropriate filters for the DTMF frequencies only built in
DT and other tones are quite different (about 440HZ modulated vs. a pair of fixed frequencies in the range 697-2000HZ for dtmf)