HI Stiam,
I'm afraid that this is not possible, or Id rather say this is not something configurable from ISDN Gw perspective.
On the general setting page of the GW you have the option "send calling number" . you can set this one to always, or "only if numeric" to make sure that the GW will sent out the E.164 calling number to the ISDN provider, but usualy this is not forwarded by the netwrok, this will be overwritten by them with the trunk number
Kind regards
Cristian