[asterisk-users] Monitoring second leg being dialed?

[Gilles]

Hello

	My ISP provides an FXS port to plug a handset, which can be used to
make free calls to (GSM) cellphones, similar to the Billion ADSL
modems:

http://au.billion.com/product/voip.php

My plan is to install an SIP client on a smartphone, so that when I'm
travelling, I can connect to a good wifi hotspot, register with an
Asterisk server at home which has an FXO card, tell Asterisk the
number I wish to dial, and have it dial out through the FXO card and
the FXS port on the ADSL modem.

Here's the diagram:

http://img844.imageshack.us/img844/3308/asterisksippstncallback.png

Problem is, Dahdi/Zaptel doesn't provide call progression, so that 1)
when Asterisk passes the call to Dahdi/Zaptel, Asterisk considers the
call "answered" although there's no actual phone connection yet, and
2) Dahdi/Zaptel doesn't trigger an event so we know if the call was
answered (and if yes, by a live human being rather than an answering
machine) or if the line is still ringing.

A so-so solution is to simply tell Asterisk to loop through a voice
message ("This is a call from Joe Allen. Please hit any key and you
will be connected"), so we know that a human being has answered the
call, but I was wondering if there were a better solution.

Is it possible for Asterisk to somehow play on channel #1 what's
happening on channel #2 while Dahdi/Zaptel is actually still dialing,
so that I handle call progression manually from my cellphone and the
callee doesn't end up hearing that odd recorded message?

Thank you.