Telkom's Billing Error Has Damaged My Credit Record

Telkom has been under-debiting my account for months, despite me calling and asking them to fix it. Instead of correcting the debit to the agreed R1200/month, they continued debiting R400—then stopped completely. My account details never changed. They eventually corrected the debit for a short while, but after I added another phone to my account, they stopped debiting altogether again. Now my credit record is affected because Telkom failed to do their job. I need urgent help and a formal letter confirming this was their error, not mine just like their first letter to me. I’m tired of going through this over and over again. This has gone on for too long.