You may have a spend cap or an international call bar.
Call customer service to check.
Numbers here Guide: Coronavirus Community Help and Support
and here http://www.o2.co.uk/contactus
Advice from an O2 Guru to get through:
If you call 202, when it comes to the reason you are calling stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
I'm afraid you have to keep trying.
They are still very busy.
Try the other numbers in the link also.