Hi: you need a specific adapter cable, and the Plantronics cables are device-specific (different cables for different brands and models of phones).
I believe this is the correct one: PL-38099-01 U10P-S, but I suggest that you contact Plantronics and ask them:
1-800-544-4660
Also, you need to sign into the OBiTALK portal, click on your phone, and get into expert configuration mode. See my screenshot. You need to set "Preferred headset device" to "RJ9 Headset". You may also want to experiment with the volume and gain levels, too.
Note: To edit a parameter's value in OBi Expert mode, you must remove both checkmarks to the far right of the row you want to edit. Scroll down and click the submit button, and wait a few minutes for the portal to remotely make the change and reboot the phone.
If you can't get it to work via RJ-9, you can buy a Plantronics "audio processor" cable, that has a DAC to convert the headset's analog audio to digital over a USB connector.