Incompatibilities exist...
All Bluetooth versions (including 5.2) are backwards compatible all the way to Bluetooth 1 as part of the standard... Assuming all devices strictly follow the Bluetooth protocol standards. So the issue isn't directly related to the Bluetooth versions, although not all manufacturers implement things strictly on the standards. If I was a betting man, I would be more inclined to believe that the problem isn't the phone as much as the headset.
We have seen a small number of compatibility issues with BT headsets not intended for phone applications, or really cheap ones (I get that doesn't apply here), with Mitel 69xx phones... Some work great and some do not, and price or quality don't seem to be the contributing factor in general. I kind of assume the headset is trying to negotiate a better quality codec or protocol and the phone is refusing it as unsupported, but the headset won't agree on a lower protocol, so it gets stuck in a loop. I don't know this headset specifically, but in some cases they have setup software or app that can allow you to enable/disable specific protocols or codecs which might just take some playing around to get working (if you device supports that).
In general, we only recommend Plantronics/Polycom, Jabra, Sennheiser, or Yealink headsets with phones, and anything else it's "unsupported and at your own risk".
