What finally worked for me on Windows10 was:
> Control Panel
> Hardware and Sound
> Devices and Printers
> Right click > Properties on WH-1000XM5
> Services Tab
> Turn OFF "handsfree telephony"
> Apply
> Turn back ON "handsfree telephony"
> Apply
Then in settings > devices > Bluetooth & other devices, I could see the devices changing from "connected music" to "connected voice, music".
Hope that helps somebody else.