Botched cancellation of Phone contract

I phoned on the 1st of October 2020, to cancel my contract ending on the 1st of November 2020, I confirmed that the agent didn't require a cancellation email from me, and the agent confirmed that the contract is cancelled. Now on the 1st of December the number was recharged again, and I have to pay the contract amount, for a contract that I already cancelled on the 1st of October 2020. I'm very upset about the very bad service of the Cancellations department.