airtime *******!!!!

i have been loading airtime almost every second day and i have also been buying the whatsapp bundle. i loaded R50 last week and all of a sudden, my airtime had disspareaded. i have lost count how many times i recharge then my airtime vanishes. i use wi-fi at work during the day and my data is off most of the time. how is this possible? its not the first time i have complained to Cell c about this and its happening again. i wsas just told to switch off my data the last time. i dont have any other apps on my phone except for whatsapp. i even deleted my banking app but still, i load airtime just to have it gone!!