Suspicious Incorrect Billing

I am currently on contract with Vodacom for a Cell phone and 3G. Both contracts are for 24 months. I have got no problem with the billing of the 3G, my only problem is with the cell phone. <br> <br> I have instructed Vodacom to set my voluntary limit at R 1200.00 and I have been receiving invoices of +- R 1800.00 or so depending on my usage for that particular month. <br> <br> Recently I have been receiving invoices of R 2300.00 to R 2500.00 and now it doesn't make sense as to how does Vodacom arrives into such figures. <br> <br> If you could clarify it for me so that I could understand how does this billing works. I would really appreciate that. <br> <br> I am looking forward hearing from you. <br>