4G and wi-fi calling is a feature you enable through the handset, the BT network supports that feature, check your instructions for the phone and enable, that's it.
You can find out how to turn it on here
When it's enabled you should be able to see it at the top of your phone in the notification area. Although be aware I have it enabled and it works, but both the BT App and "My Account" page say it's NOT on.
Hope that helps
Thanks. My phone does show that they are both enabled and indeed it seems that they are. Its weird that they app and my account doesn't show they are though. I guess that was what confused me.
I have the same problem on an iPhone XS, both 4G calling and WiFi calling work but both show up as not compatible on the Phone and Sim section of My Mobile on the My BT webpage and as NO under Phone Connectivity on the BT Mobile app.
The phone's IMEI number, Sim serial number and all other information are all correct, so its very odd that this cant be sorted out.
Mine is the same. Wi-fi and 4G calling work fine on my XS, but show not available in my BT, so not a big problem.
Do you have VoLTE enabled ? I'm sure I've read it interferes with Wi-fi calling, try switching it off and testing again.