on 11-01-2019 09:02
A number of things to try
Open settings on phone and check you have the correct message centre on your device
Pay Monthly +447802000332
Pay & Go +447802092035
Also take your sim out and clean it gently, reboot the phone.
Finally check your network status here https://status.o2.co.uk/
on 14-01-2019 10:51