You'll need to call O2, borrow a phone, use a landline or Skype
Numbers here Guide: Coronavirus Community Help and Support
Advice from an O2 Guru regarding a way 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 an advisor.
Press Upgrade, Lost/Stolen or Fraud if you have to.
Veritas Numquam Perit
You already have a thread going here https://community.o2.co.uk/t5/Pay-Monthly/About-my-payment-to-o2/m-p/1414809#M218623