Here are the notes for all this to work. I don't know if it's written somewhere so I put it here.
1. Jabra and Plantronics require their software installed.
2. Plantronics software supports only one headset connected at a time while Jabra can handle multiple headsets.
3. There might be some old headsets which are not supported by their current software so they won't be supported in 3CXPhone. I know at least one such headset for Plantronics.
4. You need Demo, Pro or Standard license on PBX
5. For headset to work you should select it's device at least as audio "Speaker" device
6. You can have two ringtones simultaneously if you leave audio "Ringing" device set to your PC. Now the PC and Headset will ring together.
7. If your headset becomes unavailable 3CXPhone will switch to Primary Driver. It's not possible to switch audio devices during the call. If afterwards your headset is available again it will be selected.
8. There are some nonobvious features of headset buttons. For example on some headsets long button press or double button press can cause some special behavior.
9. Don't press headset buttons for fun
When you press a button headset goes "online". Some of them turns off automatically after some seconds, some don't and 3CXPhone tries to put them "offline" for itself. Anyway use buttons only when you want some actions.
Hope that's all