Neotel billing ***********

I have just received a bill from Neotel claiming my account is R3000 in arrears. This impossible since my account is capped at 5 gigs and I pay a set fee every month which I've never failed to pay. The worst of it is no matter how many times I call the call center or after 4 emails, no one can get back to me as to why my account is in arrears. This is very suspicious!! I find Neotel staff are extremely *********** and under trained. I've asked them to investigate and no one has. My contract ends in Oct and I plan never to deal with Neotel again! I'm not going to hand over R3k if no one can tell me why I owe the money!! I pay R148 per month so I have no idea how they got to that amount in first place. It's outrageous!! I've only twice used 6 gigs in 2yrs, usually I'm only using 2 gigs a month.