STILL BEING OVERCHARGED FOR 5 MONTHS

I am still, after 5 months, being OVERCHARGED for a contract. I upgraded my sons cell contract and took a contract for R149.00 inc Vat per month, since NOV I have been charged R160.00 ????? now I ask you how? The branch has been assisting in trying to correct this but MTN just gives a fig.................................... then they say it has been corrected, and you have a few hours / days to see if the problem has been solved....................HOW if you only get a account once a month. Is MTN going to correct this and credit my account???? Never mind that my own contract has been incorrectly charged for the last 2 years

I would also like to know: the invoice sent on 12.04.18 was for USAGE DURING MARCH - surely this invoice should have been invoiced at 14%