Unauthorized debit order debited in 2019 backdated to 2017

I have a contract phone with MTN for the r years and in November 2019 my account was debited with, R1200 by Mtn whereelse t monthly installment is R606, when i called Mtn i was told that t extra amount was for 2017 which is related to some bill that MTN forgot to debit. When i asked. After 2 years without any arrangement with me why did thy decide to just debit my account.and each and every month since i have had t contract for years with them they have been taking money via debit order without any defaults and secondly i have upgraded with them and all this was successful and no notification of this extra transaction that they claiming from my account. If there was why was this extra amount not deducted long ago and why was an upgrade, airtime allowed to pass if there was a problem. I asked that t matter gets escalated to the manager only to be placed on hold for 30min+ and advised no manager is available and one will call me back. To date no one has bothered to give me a call however MTN still have t decency to still debit my account which i still dispute. I am very unhappy and want to cancel my long service contract with MTN and want an explanation for the unauthorized debit order amount.