I was wrongly billed, instead of resolving the matter my account has been handed over to the collectors

I found myself in a very unfavourable situation when my account was incorrectly billed in May and for June as well, and up to this day the matter has not been resolved. This has also started to affect my credit-worthiness as well. I went to the MTN store in Sasolburg (REQ ********** ) and the store wrote to the customer support as follows:

Good day

Please assist as customer got two high bills from MTN for packet switch data call charges but contract has a SUL on it that was set to R1.00 initially when contract was opened.

After the forst no;; (R3090.92) was received SUL was set to R0.00 but customer still got another high bill (R4867.61). I have yet again set the SUL to R1.00, please ensure the customer does not get another incorrect bill and that no EXTRA COST must be occurred on the customers number, his daughter is using the phone and MUST BE ON TOP UP.

Please make sure SUL is set and please credit amount on his MTN ACCOUNT which was charged incorrectly as SUL was set with initial connection

Please assist urgently in this matter - customer is furious