Deducting more then what my contract states

Good day. I have a contract with Vodacom. My payment for every month sonR384 per month which is affordable to me...all of a sudden I got a call from them I was at work that time couldn't hear what they said but I dropped the call. Then end of December I got a message they Gona deduct 599 every month... I went inside the store at Sanlam centre in Parow where the lady gave me customer service number. Becoz I got paid earlier in December by the end of the month there was no funds available for them to deduct. So come January they took 384 for December and then 599 for January all together in total it was 1084...so I tried to call customer service but I was told my account has been blocked but they already took my money. I want to cancel this contract with immediate effect and I'm gonna reverse my money because how can they just take my money for granted like that without consulting me and then have the decency to block my calls. I'm no longer using that SIM card I have to use my old one now because of them