MTN refusing to refund after overcharging

I had an experience to forget with this MTN company. I took a contract which they advertised for R689. However, when the bill came, I was charged R1,112.98, which is R423 more than advertised, and it is for 36 months. This means my total bill after 36 months will be R40,067.28 which is R24 804, which is R14,940 more than the advertised amount. I first queried this with MTN then was referred to Mondo because they manage online applications. Mondo took more than a month to resolve the query, and they just came back to say that my query is invalid because they captured the amount correctly as R689 and the issue is with MTN. I then went back to MTN, and finally, one of their call centre agents acknowledged the error in the billing department. So since 19 July, I have been trying to get this error to be corrected. I think MTN is using a trick of just opening a query with a reference number then tell me to wait for 3 days, but they close that query within an hour of being lodged. When I call again, they repeat that process, and they don't resolve the issue. I had the following reference numbers for the same query, and all closed without being resolved.

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Today they gave this one ***** and told me to wait three days for a response. Why must an active query or escalation go to the back of the queue?