Terrible service from Cell C

I phoned Cell C at the end of February to change my contract to a top up contract. They assured me that this will change as of the 1st of March but again this month I get a bill of allmost a R1000. I think it is only fair that Cell C reinburses me for all amounts above the R100 that my top up contract is suppose to be capped at and that my contract be changed immediately.