Thanks OBT. I believe I have isolated the issue to the line voltage. The USB interface works on CO lines, and a standard analog set with CID works when hooked up as a station. For this application, the USB interface is connected directly to the PBX as a standard 1-pair SLT, not as a PT.

I am pretty sure there is no resolution to this problem, but, given the depth of experience on this board, I figured that if there was an answer (like some obscure PBX option), this would be the place to find it.