Incorrect balance on my statement

I tried to settle my loan account last year September and the incorrect amount was settled and at the end of the month i was still debited although the amount that was left was less than the debit amount I still got debited the full installment. I spent hours on the phone with call centre trying to rectify that and the agent agreed with me that I was debited an incorrect amount and logged a query for a refund for me but when the refund was done I was short refunded and when i called to understand why there was no explanation and agent told me she also couldn't understand was therefore logging a query so i get refunded the rest of the money and I never heard from African bank after that. I continuously made calls which were no longer being picked up when someone picks up a call will be dropped after holding for almost 30 minutes and i decided to reverse the transaction as I couldn't get help anymore with the hope that someone will give me a call to check why did I reverse the transaction but no one did until today. Today i logged in to my African bank profile only to find that I now owe 8700?? How on earth did I end up owing that much in less then a year when my balance was less than 3000? Please review this as this was not my fault in the first place. If you check the system notes you will see I literally tried to get this resolved but African bank did not play their part therefore cannot be my fault. I'm still willing to pay the balance that was left but it cannot be fair to put that amount on me when i literally tried with no success to get help with resolving this.