Overcharges and poor service by MTN

I have been dealing with an issue with MTN since October 2020. They overcharged me on my August and September invoice for a promotional fee that I did not take up or know about, via my debit order. I have tried to get them to credit me at this time, to no avail, despite numerous emails and phone calls. In December 2020, my contract came to an end, but I refused to renew my contract because of the outstanding amount. In January 2021, MTN suspended my services because they insisted that my account was in arrears. In turns out that they changed to a new billing system but did not transfer or assign my payments correctly. I explained they situation telephonically again, and I emailed a spreadsheet with the amounts charged, and the proof of debit orders paid and what was due to me. My phone was activated and my query was logged in. Between then and now I have had my services suspended several times; have had to call in and explain to several different people about the same issue, email an updated spreadsheet of the amounts that could be deducted from the amount owed to me and MTN would still be owing me funds. This last weekend I called in beforehand to see if my account was still remaining active as the issue had not been resolved as to add insult to injury, the billing department was looking at the wrong number (last digit changed) and wrong account information. I was assured that my services would not be suspended, yet the very next day it was suspended and I was sent a final letter threatening legal action. MTN, this was a long weekend. I am a healthcare worker, who consults for the government and WHO. I have online business that requires OTPs to come through. I previously had to stress about a SMS coming through regaring a vaccination appointment while my services were suspended. You have cost me time and money and productivity. YOU STILL OWE ME MONEY!!!!

Can you please resolve this issue before I take legal action? Can you resolve it this week? How long does it take one person to allocate the correct payments to the correct invoice and account and refund an overcharge?