asterisk users - Mar 2005 - Call through. with 2xT1 .configuration

Hello all,
It 's dificult to explain; The system I need is an box option (based on *), 
that I would add to an existing PABX (ie: Nortel with 600 ext). 
I need two E1/T2 card to plug the system between Telco (FT) and PABX (Nortel)! 
One card for France Telecom Side (E1a) and one other to Nortel Side (E1b). 
---------                 --------                ----------- 
Telco FT |----(E1a)------|System X|----(E1b)-----|PABX Nortel|------>600 ext.
---------                 --------                ----------- 

The existing (Nortel) PABX must run like before the system box is plug, 
without any modification on it. 

The System box intercept only incomming call on special DID/SDA ie: 4000. 
Then a prompt ask the extension to be reach, and the system box call this 
extension via the E1b link through the Nortel PABX. 
All other incoming and all outgoing call pass through the system box,
transparency. 
Just a small system is requiered, I think, because there is only 30 
simultaneous calls. No VOIP, No voicemail, No SIPphone, NO Extension. 
The goal is to unsucribed average 200 DID from France Telecom with this system.
I search some explains or samples to configure * to do that. 

Regards

Florent