Cell C and the airtime issue on a contract phone

I have a contract with cell c, I have a l have set a limit for the airtime , the only thing that I do with my phone is a whatsup and am on the R5 recurring whatsup thing, but out of the blue I receive the messages saying am now left wit R164.00, then after 2 hours another message am left with R 50.60 and when I call to my surprise the amount that was sent to me doesn't correspond with the one the consultant see on the screen , this is disturbing and irritating. I asked them to check what's chowing my airtime but oops no respond . this is unprofessional.