Cellphone contract from hell

In May I bought a cellphone on contract from MTN Carnival Mall. I signed and agreed to pay R349 per month. Every month I'm being charged more than the R349. In June I was charged R28 more, in July R20 more, and August R188 more. I called MTN call centre, and they told me about itemized billing. I never agreed on any itemized billing in my contract. My question is, how can i be charged more than my monthly premium that we agreed on, while the data and minutes that MTN gives me is the same every month? What an exploitation.