Telkom Fibre overcharging me AGAIN

I signed up for a Fibre account with R399 charge per month but every month I am overcharged and have to keep calling to correct my bill. I have ended up paying the higher amount every month and when I call so they correct my bill they say they will not because I must pay the amount in arrears but I've been overcharged and Telekom actalually owes me a credit.. can you please fix your billing and charge me the amount agreed on and credit my account asap.