Debited for cancelled contract

I cancelled my contract on 8 December 2017 and was assured twice by retentions team that my line was transferred to a prepaid line before 20 December 2017. Yet a debit order went off yesterday for the cancelled contract. I need an explanation and refund as I was told by the agents that there would be no further account deductions since the contract was cancelled. Alternatively, I'm reversing the debit order today as it was unauthorized.