MTN billing system

I have a contract with MTN starting 11 April 2022. The first deduction went through on the 15 May 2022 for the amount of R852.95 On the 15 June 2022 I was however billed and my account debited with an amount of R9699.47. I then called MTN on numerous occasions to sort this out, but I have receive no assistance nor explanation of why i was billed this much. MTN is now threatening to effect my credit status with unpaid fees, even though they have not resolved my issue nor communicated the results of my query to them.