asterisk users - Sep 2006 - ISDN HFC card cannot 'detect remote answer'

Hi all,
	I have a box with an ISDN HFC card (1 BRI) connected to an Italian 
ISDN, the card is using zaphfc driver and it's receiving and 
originating calls quite regularly.

There are some numbers (mostly toll free numbers) that I cannot 
connect to, here is what I get from the CLI:

     -- Called 5/803868
     -- Zap/5-1 is proceeding passing it to SIP/184-11da8730
     -- Zap/5-1 is making progress passing it to SIP/184-11da8730

But it never foes any further, it just hangs like that waiting for the timeout.

My guess is the problem is related to the number being 'toll free', 
It seems to me that card isn't 'detecting a remote answer' and is
not
opening the audio channel.

Does it make sense ? I'm new to ISDN protocol ....

BTW, if I place the call with an FXO cards it works very well

Here is my system configuration, hope it helps
- Asterisk version is  Asterisk 1.2.11-BRIstuffed-0.3.0-PRE-1s
- Libpri version 1.2.3
- Zaptel version 1.2.8
- Linux distribution: Gentoo

Tnx in advance for help

Regards

Edoardo