I think the phone spec is at least 50mS tone on time and
a 50mS tone off time.  200mS is too long.
   Dedicated chips for DTMF decoding are usually in the
40mS on/40mS spacing value.
Dave

>>The tone is detected within about 150ms (I don't know the complete
>>spec, but I think tones are supposed to last for at least 200ms).