There are two outputs from the PBX possible using phones1's setup either as a U interface or an S interface. Polycom FX and VSX units have an S/T interface built into them so if your Polycom and the PBX are under 500 meters or so from each other you can run the S/T card with the controller card. If your units are beyond that in distance you will need to use the U/T interface cards at the PBX and then add the Quad BRI and Quad NT1 modules to your Polycom at the endpoint to connect.
I would run a DID to the unit for inbound and make a dedicated outbound route for the Polycom, set the min and max in the PBX to limit how many channels it can grab for each call.
I am actually using a stand alone device made by Patapsco call the liberator, it shares PRI to BRI as well. I have 2 FX units and one VSX running off it and they are installed in 3 different capus buildings under 500m cable distance so I don't use U/T and NT1 boxes.