Increased debits based on assumptions and no way to rectify

Having received a monthly bill of R 1086. 26 whereas my contract is capped at a monthly premium of R528 per month, I phoned MTN to enquire. I was told that there was an unpaid bill from April of 2015 that they picked up on and will be debiting my account. My account is on debit order and there has never been a missed debit. I confirmed this by opening my banking app and going back to 2015 to confirm that all debits for that year went through and for the full amount. The consultant had raised a ticket. I must admit that Nolani was very helpful however, I could not accept that MTN had only now discovered a “bounced” order and decided to debit my account without consulting me and investigating further. Since my contract is up I decided to cancel as I do not wish to continue doing business with an ********* service provider that makes assumptions and charges their clients based on these assumptions. I asked to be transferred to retentions. After 5 min, the call was answered and I was told that I could only cancel my contract in August, and the only way to do it is telephonically. I asked for a buyout option via email and was told this could only be done telephonically and was put on hold while the calculations were being done. After being on hold for almost 10 minutes, I decided to give it up as clearly there was no interest in assisting a client. The total call duration was 43 minutes which is disgusting in my opinion. My account has still not been rectified and I still do not have the figures to cancel. I would like immediate confirmation that my bill has been rectified as this is clearly an error on your end and has nothing to do with me. I am really disappointed in MTN 's service or lack thereof . I had expected more from a company of their size.