Cell C is ripping me off

I have an existing contract with Cell C, the contract is an open line with R100.00 free airtime and i made my limit R100.00 which makes it R200.00 for me to use monthly. For the month of April i did not get the free R100.00 airtime instead they only gave me the R100.00 limit amount but they charged me the full contractual amount plus the R100.00, i called in to customer service yesterday and i asked the agent to calculate the calls that i have made and check if i have used the R200.00 and she said that according to her calculations i hadnt. so where is my airtime? and why am i being charged for a service im not recieving? i want out Cell c is not transaparent and if you are not careful you will get **********.