BT's position is, or at least was last time I looked, that only BT supplied handsets are "supported". But many users are happily using non-BT supplied handsets with WiFi calling. I've had it working on a Nokia 8, Samsung S5, S8+ & now an S10+, but I did have to flash the S5 as it was an O2 handset with their firmware.
There was a very long thread dedicated to WiFi calling, there may be some help there if you can find it.
Edit
https://community.bt.com/t5/Archive-Staging/Wifi-Calling/m-p/1888469
But it's been dormant for a couple of years.
