Cheating practice from Vodacom

I am very disappointed with Vodacom. My problem started in July this year when Vodacom debited R713 from my account, R613 the following month in August, R613 in October, R613 in November. I am using top up the contract, I consented to debit of R444.99 per month. I wrote E-mails to them, asking them to take me to debit of R444.99 per month. Vodacom is continuing with debit of R613 per month. What i want from them is to take my debit back to R444.99 per month. I never upgraded to R613 per month contract . i thought all along that Vodacom are professionals. How come Vodacom made 3different amount of debits in different months for one contract? My invoice no is k***, ACC no is NA051516, I upgraded my contract in October last year.