Unauthorised debiting amounts and changing of debited amounts

I've had a contract with Vodacom for 300g per month for less than R300 a month now they keep billing me more than R300 a month.Their billing keep changes every month.They decided to stop debiting without my concerned and reported me to credit bureau that I've missed my payment twice while they are the once who didn't debit and instructed me to go to Vodacom store to pay which I did,after paying they go and debit close to R1000 from my account.When I call to cancel my contract they want me to pay R1400 saying it's early termination fees. Checking their fees they have now changed my monthly amount to R631 which was never that amount.Vodacom contracts are a **** and they treat customers so bad