22-12-2018 17:59 - edited 22-12-2018 18:01
A weird one. Though it could be a problem with your sim
Take it out, clean it gently with a lint free cloth put it back and reboot the phone.
Or you could get a new sim from an O2 store?
You could also try a manual network search, choose one that isn't O2, Forget that network and and try O2 again...
For info we have a guide here on network issues
on 24-12-2018 11:17