Triple debited my account

If there's one thing I regret then it is taking a contract out with Telkom. After battling for months to get my phone I got it mid September after they debited my account in August already. Then they debited my account in November for the amount of R499. Today they debited my account for R1520.64???????? How on earth do you go from R499 to that amount. I want an answer and my money BAC****!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!