> I would like to use this circuit in a pic-controlled 
> telephone dialer.

A long long time ago I made a dialer. LM567 decoders and
timing s/w were used for detecting the various tones. Finding
the difference between a real person and an IC person was
tricky. What we decided on was to assume the answerer was
human, if it was an IC do the spiel anyway, and look for the
disconnect tone either way

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