Incorrect billing not being corrected!

Took out a contract in Jan 2024, with a trade-in of my old Samsung phone to reduce the cost of the contract, they have incorrectly billed me over the last 3 months, did however pass a credit for 2 and said would correct the rest to no avail. Absolutely no help from the MTN shop in Clearwater Mall, just say they attending to it. Only response from '***' is when you copy ICASA, even they dont resond to mail. MTN Ref no f58fc0a3-979d-4efb-8f92-accd***b