old account which i have no account number for how do i close it??

I have had a bill for ID mobile for the last 5 years, I have been with O2 years but took out a contract with the carphonewherehouse when I broke a brand new hand set. I have just discovered this bill is now still going dispite asking for it to be cancelled twice. How do I get a refund. I cannot register with my mobile number or date of birth because it's not recognised. I can't find my account number either. I need a refund but I cannot call anyone due to lockdown and the online chatbot just keeps telling me to register.