INCOMPETEND SERVICE

I have been customer for MTN for more than 30 years but the treatment I am getting from them is not good. In November I have upgraded my contract with them on black Friday special whereby the deal was R199.00 for 50 gig only no open line which my son is using, they deducted R199.00 only in January from they have deducting the R718.00 when I went in to the branch to query they told me my son could open the app on the his phone to open the line, I have asked them how can their system allow anybody to open the line make calls without my concerned, I was given ref no *** that they will resort the matter nothing was done till to date I am paying the R774 every month . I told them I do have contract with Vodacom for my other kids ,Vodacom will never allow anybody to open or go into my account without my concerned they will call me but with Mtn my son can increase limit and their system allow that. I told them I want to cancel the contract because s not what I signed for they telling I must still pay couple of thousand's so please can someone help me on this matter as urgent please