MTN is overcharging her contract customers and they’re no documents given about why they’re overcharging

MTN is very **********. My mother has been using MTN for all her life and she has an ongoing contract with MTN for years but they suddenly started to overcharge her for no good reason. The first time she was overcharged an amount of R3800 mind you the contract is only R900 per month, when she enquired at the MTN branch in King Williams Town (Stone towers shopping centre) to be exact the staff was so unprofessional and the feedback she got was very unclear and so absurd, she didn’t even receive a document about what they’re overcharging her for. Fast forward this month they’re still overcharging her but now the amount is R7800. Bare in mind even though she didn’t know what she was overcharged for in the first place she made an arrangement to settle the bill of R3800. Honestly I don’t know what is happening at MTN