A couple things could be happening here.
First, could be that the PBX is not providing calling number detail in the phone display sent to PIMG. You may be able to test for this by pulling a line out of PIMG and putting it into an Optiset phone.
Second, if the PBX is providing the detail, it could be that PIMG is failing to parse the display correctly. To see this, you'd need to enable and gather some traces from the PIMG. Your best route for that might be to open a TAC case.
-Eric