Incorrect Billing

I have recently migrated from smart plan 50 to smart plan 100 on my cellphone contract and Telkom has somehow managed to mess up my monthly fee. Telkom has failed to assist me with an incorrect invoice amount on my statement for the past month. I have been billed incorrectly on random dates for random amounts and charged for unpaid debits if the amount was more than I had in my account. The last amount debited from my account is R1500, which hasn't been returned to me, and even Telkom agents don't know the reason for the debit. Further this has a negative rating on my credit record, as I recently at attempted to purchase a car on finance, due to the recent Telkom incidents, I was unable to.