Incorrect Billing Every Month! Pathetic service!

My fianc? has a contract with MTN for our 2 sim cards, a internet usb and a tablet. We have more than once gone in, and had it confirmed that the limit is set on 0 - yet every month we owe more than the agreed amount. Calling into the call centre is useless...we got advised that the contracts went over their given min or data amounts and that the payment is correct. We then went into MTN and the guy showed us the limits on the contracts is still in tact and he cant see how the accounts were billed extra. Why is there a limit set if MTN cant assure you that what you signed for is going to happen? MTN is by far the worst company to deal with. One thing is for sure, this is the last contract that we will take out by MTN. Now we must produce the original contract and take it up with their complaints? Should our contract not be on the system for them to check? I just feel that the customer service you get is pathetic, in 1 shopping centre we went to 2 MTN shops, to be told 2 complete different stories about our account? Not one month can we budget properly for our MTN account, as it differs completely every month!