Yeah, the prices are in the BMW/Mercedes Benz part price range - precious.
You'll need to go to the OBiTALK portal, click on your phone and get into "Expert configuration mode". Go to User Settings-->User Preferences. Scroll down and find the EHS section. Remember, to edit any parameters, you need to un-check both boxes to the right of that parameter. Change EhsEnable to Plantronics. I have a Jabra, not a Plantronics, so my volume/gain settings may differ from yours, but I'm including a screenshot to give you a start. Note that the settings are confusing, because there are separate volume/gain settings for the RJ-9 headset, the USB-attached headset, the built-in handset, and the Bluetooth headset. You can also set it to prefer the headset when pressing the call button on the phone while it is on-hook (otherwise, it defaults to the phone's speaker).