Hi,
I think the basic T1 CAS protocol doesn't support sending the calling number independently from the fact you are connecting telco or a PBX.
You may want to check this with the PBX guys (if they are willing to help). Or ask them what would be the PBX setting to see calling number if they were connecting the PBXs over a leased line, and then configure the router accordingly.
I understand that there maybe license/hardware issue to have the PBX do PRi, but that would be by far the easiest way to a very complete and robust integration.