over charge

Ever since I took contract phone, they never billed me the correct amount. I lodged a complaint with them since October2015 about the matter because I am supposed to pay R639.00. Oct/ Nov 2015 I was billed R913.39. They promised to fix the problem and to refund me but it never happened. DEC JAN/FEB 2015 they decided to charge correct amount which is R639.00. I was like ok my matter is now resolved. Again March overcharged me R796.00, immediately I contacted them, they told me stories that I subscribed to receive data . I am quite sure that subscription was never done with my phone because I am the only person who have access to my phone and is always in my possession and it does have a password . I have requested to cancel which they did so because I received notification of cancellation. To my surprise on the 12 and 14 April 2016 again while sitting at home alone with my phone next to me, notification message came through from mtn that says I am successful re_subscribed to data. 15 April 2016 charged me R1052.89. I never subcribed to anything, Mtn subscribed my number. I don't believe in miracles, my phone can't subscribe automatically. They can't help me. Be warned.