cell c debting my account

I have a tab 4 samsung contract with cell c that is debiting on the last day each month, four month bacc wanted them to debt on the 15 due to the fact that month was on maternity leave and it was already the second month I was on martenity leave but they said they could not change the debit order on that month but only the following month and they debited on that month end to my suprised on the 19th of that they sent me a request that I need to pay immediately or my services will b suspended I call customer service going forth and back bt no one assisted me and my services were suspended two days later until month end when they got the funds on my account and all data were gone because I was receiving that month. With the last consultant I spoke to I made it clear that they must never change my debit again. I am very much suprised today when they are sending me a notice that they will be debiting tomorrow after they debited month end. I am so tired of cell c bad service and I don't want to change my account because they are not the only person debiting in my account and this means they will again suspend my service after I paid for this month.