Awefull service

I cancelled my contract end of April, and yet my account was debited every month after that, i finally stopped debit order only to receive a message im in arrears,so when I phoned one of the agents i was told that my contract was migrated to month to month, even though told the agent i cancelled it, now im told I must pay the arrears amount,for them to cancel it, and investigate further, and if its their fault they will credit me, how does that make sense, and yet they say calls are recorded