Can you guys recommend a headset that you know for certain works with the polycom 331...
Do we need the lifters?
Figured it out! The RG9 to 2.5mm male adapter was bad, two of them in fact. I found a brand new one still in its bag, plugged it in, and walla, two-way communication!
2200-11095-002 Rev A was the culprit.
Now hopefully that explains my frustration with this. I had tried (what I thought) was everything and followed all the guides and you guys advice.
Makes sense now why some other setups were working and these new headsets were not (adapters were not new).