Looks like BT may have resolved this now. If you are still affected, turn on Airplane Mode and then turn it off again.
There definitely was an outage of some description because I'd tried both Airplane Mode and rebooting the phone a good few times today and it made no difference then, but when I tried it again just now it did work.
Working here as well again. Airplane mode on and off great tip
I still have this problem. Hasn't resolved for me.
None of the re-boot options have made a difference.
I guess it is worth checking if 3G coverage has been shut off in your area!
Mobile Network Coverage Checker | 3G, 4G & 5G | BT Mobile
or I guess
Coverage Checker | Check Coverage and Signal in Your Area | EE
and click on '3G' not '5G', '4G', '2G'.
That should direct you to network settings on your phone. I guess they are hoping most are set to '3G/4G Auto' so we won't notice.
Forcing 4G may help?
I have SMS messages and 4G data OK, but not voice phone calls ...