🚀 go-pugleaf

  Perhaps the simplest way is to provide two bandpass filters,
one for the low frequency tone group and the other for the
high frequency group, and count zero crossings to determine
the frequency of the tone in each passband.  The tone frequencies
are (if I remember right) 697, 770, 852, 941 for the low group
and 1209, 1336, 1477, 1634 for the high group.
  A much better way is to supply a +/- 1.5% bandpass filter
for each tone frequency.  The filter outputs (of the two tones
or of the high and low band) should be within 8 or 10 db of
each other, else it is a reject.  Also it is a good idea to
compare with the band from 350 to 620 Hz to differentiate
from progress tones and speech.  An amplitude threshold at
about -35 dbm will reject false alarms from background noises.