MTN ******* debit

I received my MTN statement reflecting an amount nearly double my monthly contract total. Upon querying this (29 January 26 minute call) I was told by the service "champion" that due to a system error, my debit order payment at the end of June 2020 was not reflecting. I have had a contract with MTN for around 20 years, with the debit order running off the same bank account for close to the same amount of time. I was advised to email proof of the payment to the supervisor directly. I obtained a bank statement, emailed, emailed again, have yet to receive any acknowledgement of my mails. The service "champion" assured me that the error would be rectified upon receipt of my bank statement. An SMS notification arrived on 2 February advising me of my "promise" to pay the amount in question. Another phone call to MTN (25 minutes), another email address, another acknowledgment that I do not in fact owe anything and I most certainly have not agreed to any debit. SMS notification today, 5 February, that the amount has been debited from my account. Another 17 minute phone call to MTN, no resolution, and you expect me to follow up in 5 working days regarding the money that you have effectively ****** from me. ***********, lying *******.