Today came the Official Update to Android 11 on my Samsung Galaxy S20 Ultra. How sad and surprised I was that after the update the WiFi Calling function disappeared !!! On Android 10 it was and after the upgrade to Android 11 it disappeared !! О2 UK please check it !!!
Have you tried the old "reboot with airplane mode on" trick?
I'd also check both call and network settings as they may have moved it to a different menu.
It did move on my OnePlus.
Have you tried to see if it's still working as wifi and 4g calling go hand in hand on android.
I have a similar problem.
My phone is an iPhone 12 Pro and up until today I was able to 4G & Wi-Fi call, now when I make a call it changes from 4G to 3G. When I go in to settings to check these services they are off and when I try to switch them on I get "cannot activate 4G/WiFi calling, contact O2" I also cannot change my visual voicemail greeting. When I try to change greeting I get a message "The operation couldn't be completed. (com.Apple.mobilephone error 1035.) I've googled this problem and have followed the steps it advises to do but with no luck, I therefore believe this has to be a network error somehow.
Track usage, upgrade and get support with the My O2 appDownload My O2