You can start by downloading the Plantronics Hub software version linked here: https://www.3cx.com/docs/
Then run any updates that it may ask for, and reboot your system. Following the guide above, it instructs you to select the headset in the 3CX app.
After this you should be able to answer calls. The 3CX client does not show up in that list, but once detected it will show up under SDK (only while 3CX Windows app is running)