> From: XARA Telecomunicaciones [mailto:xara@velocom.com.ar]
>
> Hi,
> I'm developing a PIC based telephony rate system
> I need to identify when the called party pickup the phone
> any Ideas?

I think you might have a little trouble here.  Generally, there is no signaling
to the calling party that the called party has answered the phone (unless you
count "hello").

There are all kinds of call progress signals that indicate that the called party
has not answered the call, and maybe you can use their absence as an indication
of an answer.  Unfortunately, most of those signals are in the audio band, and
probably need to be analyzed in the frequency domain (maybe with a dsPIC).

Just off the top of my head, the following signals mean that the call was not
answered:
- Calling party hangs up
- Ringback (ringing caller hears)
- Busy
- Fast busy
- Intercept (tones followed by message)

There are also some nasty scenarios that you need to be able to handle to get a
good count:

1. Conference calling (three-way calling).  Here someone can call you, and you
can call someone else without ever hanging up.  You will be billed for the
outgoing call.

2. Call someone, then receive a call on call waiting.  Switch to the other call,
and some time later the person on the first call hangs up.  You will only be
billed for the time the first person stayed on the line.

3. Receive a call with reversed charges.  You will be billed, but didn't make a
call.

4. Receive an automatic call back when a busy number that you were trying to
reach became unbusy.  I think you get charged for the call back.

I'm sure that I left several conditions out.  I know that in the good old days
people made equipment that did what you're trying to do, but I don't know if I'm
over looking something, or the network has become more complicated since those
days.

-Mike

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