Can you log in online https://mymobile.o2.co.uk/ ?
You can reach the Payment Management team on 0800 902 0217.
The opening times are:
Monday to Friday – 8am to 8pm
Saturday – 8am to 6pm
Sunday – 9am to 6pm
All ways to pay here including how to set up a direct debit to avoid future issues Guide: How to Pay Your Bill (Contract)