Over debited account

I have been with Telkom mobile since February 2016 and every month I have been over debited! I have an installment of R550 yet am always debited R1000's over! This month they debited R10300!!!! Each time I have called their call centre and am told the problem has been escliated and nothing! Last month I was told the problem was fixed, yet this month the same thing. I am sick of paying bank charges and reversing unauthorized transactions! This is theft and I've just about had it with their awful service. If this happens again next month I will cancel my contract and refuse further payment!