Billed on airtime instead of data

Cell C has a great deal during the evening that allows customers to purchase 1Gb of data for R12.50 which can be used between 1am and 7am. During the past few weeks I've been taking advantage of this deal and staying up late to download games and software. However at certain instances after purchasing this midnight data, cell c has not deducted the charges from my evening data but FROM MY AIRTIME! This is absolutely a ******** act. Cell C handles the deductions based on their own systems, as customers we have no \switch"that we can use to change the charges from data to airtime or from airtime to data. My airtime has been totally used up as well as my normal data"