Double billing on a 24 month contract

I took out a phone contract for my little brothers phone. my understanding is that this was a fixed contract. It's the second contract I have taken out with MTN. The amount on the bill is over double the contract price... I have called the customer care multiple time and I can't get a clear explanation of the additional charges. The call centre has been absolutely no help. At this I want to speak to a manger in billing, not a manager for customer care or the MTN call centre.

This is horrible because my brother has chosen not to use his phone to prove that these charges are bogus...