MTN cancellation 'done' but still get debited month to month

I cancelled my MTN number, which my contract expired, 2 months ago. It was confirmed that it was cancelled and would go prepaid and NOT month to month. Of course I've been debited once again for the past 2 months and the number is still on month to month. I hold on call for hours and cannot get through to retentions. I have just been holding for an hour and no answer. I want a manager to contact me immediately and those last 2 payments will be refunded and number cancelled!