Extra fees on mobile account

<p>Good day</p> <p> </p> <p>I am very upset and furious with MTNSP, how could it be that I sign a contract with them for R609.00 per month which includes 500MB of data and R200.00 airtime (top-up contract) for a period of 24 months and after the first 12 months I get invoices with different amounts ranging from R700.00 to R726.00 and my airtime that I get is now R100.00? Please explain how this work?</p>