I have had something similar to this:
Incoming number, with an IDC conversion to a steering code (DSC) that then forwarded to a public trunk (ISDN) and failed.
The answer after going through the vendor and ultimately Nortel was that we were "missing" pkg 366. This package seems to consist of "patches" that "fixes" things like this. Not sure when this came in, but believe it was 25.40B and possibly earlier.
The net result was that Nortel "provided a patch" that overcame the problem. There was a lot of arguement about this, due to Nortel wanting to charge for the patch,and prior to an upgrade that we had had, the feature was working. It is believed that the upgrading vendor may not have checked the patch lists.
Good Luck