U.K. unlocked, BTU, stable Android 11.
Wi-Fi calling is showing and working fine.
Try accessing it either by pulling the Notification panel down and long pressing on the Wi-Fi calling icon or by opening the Phone app > Menu (3dots top right) > Settings > Wi-Fi Calling
Which U.K. carrier are you on?
These are my settings for Phone...
Try going to...
Settings > Apps > Phone > Storage > Clear cache
Reboot your phone and see if that helps.
According to this video... https://www.youtube.com/watch?v=RtFsDPjRsnA&feature=emb_title you need to download the O2 app from the Play Store and set up Wi-Fi calling via that. Alternatively I suppose, you could just contact O2 Customer Services.