Yah, when I looked at the Plantronics website earlier, I saw that there were two, very similar cables, with ambiguous explanations about which to use in which situation. Glad you figured it out. When you get the replacement cable, there are settings on the OBiTALK web portal, in "Expert configuration" mode, that allow you to adjust the volume, and to select the default thing to use for calls (headset vs. handset). Note that settings are in a very illogical place ("User Settings --> User Preferences"), vs. where I'd expect to find them, in the "IP Phone" section.
I'm using a Jabra headset with EHS, but you'll use the same settings to tweak your headset's audio. Also note that there are three different sections of headset settings; "Headset", which is the modular RJ-9 jack you are using, and "Headset35mm", which controls the round jack on the side of the phone, and "HeadsetBT" for Bluetooth headsets.