Overcharged bill with a query open from Jan still no refund

I had a 2 year contract on a data and modern with MTN which was around 139.00 capped for monthly which I opened at the mtn store in Cornubia mall. On several months I was overcharged and I will query it and they will sort the billing, however after lockdown last year their telephone lines were never taking me through to correct people and I could not get my query solved. Ultimately I went to a shop Jan to log a query which I did not have any response back. I went back to the shop and they told me ai need bank statements to prove all the billing which I got and I emailed to them Nhlanhla Zulu initially responded saying he has escaped the query this was around May this year and in August I am still resending emails with no luck and even their social media account they have not come back to me with a clear response on when they will be re funding me close to R17000 that I was overcharged. This resulted in me not trust companies with contracts and I'm now only buying data cash.