Today, I received an email from O2 informing me that my invoice for December is ready. However, I requested my contract termination since the end of October. I had 2 phone numbers with them, I separately requested the contract cancelation for both of them and asked to keep one to change to my new company. I do not understand why I still got a bill from O2 since I have been away from this company for more than 30 days. In addition, I cannot complain using my O2 account which has been correctly canceled.

What should I do to get this last payment refunded?