That phone supports WiFi calling so it should be fine, even with no network signal.
The fact WiFi calling isn't working and you don't know anyone else on O2 based networks with the problem suggests a fault with the SIM or the phone itself.
I've managed to sort the issue by removing my eSIM then reinstalling it. It worked immediately and my phone is back to normal again (thank God). It's awful that the Sky helpdesk didn't give me this advice. All they kept telling me was that the mast in my area was down and engineers was working on it. It doesn't take engineers 8 days to fix a problem so I knew there was obviously something else wrong with my phone.
