> >        A friend has a fax machine with a very clever feature.  I think
> >it automatically answers after maybe 6 rings, but it is always =
> listening
> >to the line.  If it hears the fax CNG tone, it goes off hook, sends
> >answer tone and continues with receiving a fax.
>
> I don't think it works that way, because such a fax machine would only =
> work on special phone systems. The answering fax cannot hear the CNG =
> tone from the calling fax without lifting the hook, and then the call is =
> already answered! The only audio which is on the line before the call is =
> answered, is caller-ID tones (if any).

If nobody answers the call, then there won't be audio on the line--true.  But
I think what he's suggesting is that the fax machine is looking for the case
where someone ELSE picks up the line and CNG tones are then audible.  If no-
thing else grabs the line before ring #6, then the fax machine answers the
line itself in case a fax is calling.

The idea here is that you can share the line between an answering machine,
fax, and (optional) person with the only caveat being that the person will
have to put up with fax calls (they'll answer the phone, then the other fax's
beep will come through, then the fax machine will grab the line and the person
can hang up).  While such switching is generally only useful for fax handling,
it may still be useful to build a box to add that ability to fax machines
without it.  Such a box could be built with a PIC 16C62x to detect the CNG
tone; the biggest difficulty would be triggering the fax machine to answer
the phone once CNG was detected.  Unfortunately, wiring into the fax's "start"
button is the only way I can see to accomplish that...