***** committed on my cell phone

Need advice on my personal MTN account account my monthly payment is R300. My latest account they send me is R10484.83 I realize this is big mistake after inquiry they claim this was a self service request so I told them this was never done or approved by me. They request a avidavid and certified copy of ID, to investigate *****. After I submit an spoke to them they still claim I owe them the money. I am a pensioner and been with MTN for more than 40 years, so I'm desperate and don't know what I can do and friendly request some advice