INCORRECT BILLING BY MTN

I have been strugling with mtn for the past two months without any sort of help, I was over charged about R1300 in my debit order in June for a spend limit I had already reduced to R150. I called numerous times no assistance! My debit order then did not go through the next month and I was told it is because am a cash payer which does not make sense as I had been all my mtn installments via debit order, now some of my mtn devices are not working and calling them for assistance is just gonna be a whole lot of struggle and going back and forth!