on 10-01-2020 13:32
Odd. I have the exact same phone as you, also from O2, and the same software version but my WiFi calling is fine. Is the option also greyed-out in the Phone App (three-dots-menu settings)?
on 17-01-2020 23:44
It's a software glitch . Proved by running the phone in safe mode.
( In my case WiFi calling and volte would show for a minute or 2 after phone reboot , then disappear. )
Didn't have time to run it down. Plus all my important stuff lives on the cloud.
Factory reset solved it.
on 20-01-2020 11:33
Might be worth popping out the SIM, rebooting and see if that fixes.