I am currently using a Lumia 640XL upgraded to windows 10. I nave a number of bluetooth headsets that I have used with other windows phones. Initially I was able to pair one of them with my 640xl and it worked well for a couple of weeksOnly, and then it stopped suddenly. I tried unpairing and pairing, I tried using one of the other headsets all to no avail. The phone has been furnished with all the latest updates with no effect.
Fwiiw, working fine on my 640XL on slow ring.