Unauthorised debits

I have a contract phone with mtn which is due for an upgrade this month. I have been paying R381.61 for since the beginning of the contract in Feb 2017. In Dec 2018, mtn debited R632.52 frm my account, double the amount I should be paying. I contacted mtn to query the amount, they said Its the content subcstription, I told the guy that I never subscribed on anything I cannot be paying for something I do not know, he promised to log a query for me and to cancle the subscription ref ********** 71, no feedback recieved from mtn. I called again on 18 Jan, I called again ref ********** 85, no feedback from mtn, my debit order in Jan again was R681.45. I NEED A REFUND mtn, THIS IS ****************. I would like to upgrade but Im scared mth is gonna continue taking my money for unauthorised things I do not know about.