> 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)