on 09-12-2018 22:11
on 09-12-2018 22:15
Change your network selection from auto to manual and try a different network.
Clean the sim and reboot the phone too.
on 09-12-2018 22:16
All support is closed now but try from 8.00am in the morning http://www.o2.co.uk/contactus
Meanwhile, try manually selecting a different network and also reboot phone a few times, but it might be a roaming bar which only customer services can help with.