[Asterisk-Users] Legacy PBX integration problem

[Dmitry Kupchinetsky]

Hi,

We are trying to integrate Asterisk in front of our existing legacy PBX:

outside line -- FXO -- Asterisk PC -- FXS -- PBX -- Extensions

Asterisk answers outside calls and the IVR asks the user to dial extension 
#.
The problem is, that when Asterisk forwards calls from the outside line
to the old PBX (using DIAL()) the PBX answers, and as far as * is concerned, 
the call
is answered. Asterisk then dials the extension number, but even if the 
extension
is busy or no one answers, the call is answered as far as * is concerned.

We want Asterisk to:
-DIAL the PBX
-wait for the PBX to answer
-dial extension number
-If there is no answer at this extension start voicemail.

Any ideas?
How can we detect when an extension is busy/no answer AFTER PBX already 
answered the call?

Thanks,

Kupchinetsky Dmitry