[Asterisk-Users] Re: call progress - what are the sticking points?

[Joe Greco]

> Joe Greco wrote:
> > More difficult is the problem of knowing when the remote end has gone
> > away.  Reversal, loop break, dial tone, and just plain silence are not
> > all that unusual as methods of detection.  In some cases, you do actually
> > need to infer that the remote has gone away.
> 
> I understand that the phone company (sometime) doesn't provide 
> information about remote hangup on POTS lines. What bugs me is the 
> simple question - how does your average 10$ answering machine detects 
> the hang up?

They don't, necessarily.

> I'm guessing the obvious - DSP and some heuristics as to what a "hangup" 
> sounds like and it sounds to me that it isn't all that hard to do in 
> Asterisk (since it's done in those cheap machines) but I would be very 
> glad to hear some tips from someone that knows a little better then me.

Answering machines get by on several mechanisms.  The ones that come to
mind are:

1) Silence detection.

2) Session time limit.

Both of these are effective at doing something vaguely right within the
requirements of an answering machine.  If you've never heard an answering
machine that's recorded a minute's worth of dialtone followed by the loud
"the phone is off the hook" tone, then I'm shocked.  :-)

Just because you can engineer around a problem doesn't make the solution 
right.

... JG
-- 
Joe Greco - sol.net Network Services - Milwaukee, WI - http://www.sol.net
"We call it the 'one bite at the apple' rule. Give me one chance [and] then I
won't contact you again." - Direct Marketing Ass'n position on e-mail spam(CNN)
With 24 million small businesses in the US alone, that's way too many apples.