I found an adapter on the internet with Polycom part number 2457-11095-002 and ordered it, only to find that its bulky shape didn't quite work with the location of the jack on the Polycom phone.
Disassembly was required in order to make re-orientation possible.
In the process, I documented how the adapter is wired. There did not appear to be any resistors, capacitors or diodes inside the adapter housing.