Actually, it's a much simpler situation than what you describe:

People don't want to make it easy for machines to call people!  I'm sure
you've had the "Please stay on the line for an important message about
<...>" phone calls.  Fax machines, modems, pagers, etc all send or
receive special tones to communicate with other machines.

There are times where such an action may be desirable, though, such as
an automated alarm system calling an emergency number.  Pagers can be
used more effectively for machine to man alerts (such as servers going
down, etc).

Can anyone else think of a good reason to have such a function that
wouldn't be equally or better served by a special reception machine, or
another form of communication?

-Adam

Borg Answering machine message:
"We are the Borg. Resistance is futile. However we are unable to
assimilate you now. Please leave your name, number, and where you can be
assimilated, and we will assimilate you as soon as possible."

Wagner Lipnharski wrote:
>
> > At 13.39 18/11/99 +0800, you wrote:
> > >Hi everyone,
> > >
> > >        I'm quite new to the list but been lurking for sometime now, so
> > pls. be
> > >gentle... I'm doing a F84 project that involves the telephone, How can i
> > >detect if the called party have answered the call i made ? What control
> > >signals does a typical telco uses to supervise the line ?
> > >
> > >All the best,
>
> Now THAT's easy!!!
> Just filter the 17367Hz special tone every telephone sends for half
> second when you answer the call... easy huh? the only problem is that no
> idiot yet develop this idea, and we still using telephones as it was
> invented 5 thousand years ago.  Ok, nobody will replace all telephone
> sets in the world, so then use the 1718Hz special tone that a central
> telephone exchange send to you when your calling party answers the
> phone, what? it is not available also? what a heck? not even a 130
> million dollars exchange system can do that? We are really doomed man!
>
> Ok, going back to the caves, take the lion aside, and put few more logs
> into the fire to keep reading this email, you just need to "guess" if
> phone was answered when the one of the "call progress*" indicators, the
> "calling" signal stopped for one of the following reasons:
>
> a) A nuclear explosion just wiped out the calling phone city, or perhaps
> the state.
> b) The very old phone lines wiring of the calling party just melted with
> the ringing voltage, and short circuited.
> c) Your phone line was cut out and you can't hear anything else.
> d) The calling party answered the phone.
>
> Now, your circuit should decide (by ...guessing) what happened at the
> other side.  Isn't ridiculous how simple it would be if the answering
> phone just send a "beep" in a special frequency when answering the call?
>
> * There are few chips called "call progress indicator" than can tell you
> if the signal at the line means "busy", "calling", "reverse call", and
> few others.
>
> Wagner.  (a grumpy person about our jurassic phone system)