I got an RJ9 to 3.5mm adapter, but I can barely hear everything even with the volume set to +6db. Why can’t this amp up enough to just power the headset without an amp? Or is “native headset support” just a lie and I need to buy a Plantronics?
The volume set to +6db makes me think you may be using the settings in the phone web interface when what is needed is the volume control on the phone which is the control just under the circular set of keys - up, down, left and right.
I have used headsets with EHS cords, Bluetooth, and simply direct into the headset port. All have worked great.
I have no answer regarding your technical problem, just encouragement that the 2170 works great with headsets but, I can speak for all headsets.
Maybe adapter is not good ?