Direct Inward Dialling Passthrough

Hello All,
I have a VOIP provider giving me a SIP connection with a 100 number dial in group. This registers with my FreePBX 2.10 box as a trunk. I then use a Digium B410P ISDN card in NT mode to connect as extension to our existing Panasonic KX TD816 phone system.
Direct inward dialling within Asterisk is working fine and I understand that. But, I am needing to have the ability for Asterisk to pass through the incoming DID number with the call going out the ISDN card so that the Panasonic system can repute it to the correct final extension. Or at the very least have the ability to send the actual Asterisk extensions number as the DID number.
I have read various post of some other people doing this, but they seemed very vague and incomplete.

If any one has any ideas or suggestions to help would be greatly appreciated.