Amount mysteriously added to invoice

I upgraded my contract a few weeks ago, very pleasant experience for once. A week goes by and I start getting MMS invoices from MTN stating that I owe R897.00 of which R299 is my monthly subscription with MTN, the remaining R598.00 had no description stating what I was being billed for. Now I went from a Topup contract, to a Anytime contract that I had hard capped at R1.00. So there is no way that I could have or can exceed my monthly data or airtime provided by MTN on either my old or new contract. I contacted MTN customer care to enquire about the charge, a week later, today, they rep**** stating that it was a charge for \packet switch data"IE accessing the MTN menu from my phone and downloading items from there. I have never downloaded anything from the MTN menu"