Airtime disappeared

Bought airtime for R10, received extra R10 dree airtime also took an advance of R5 so that makes it a total of R25. My first call was of a duration of 5 minutes and the second call was 3 minutes, to my surprise my second call got cut because airtime deplited. How so if i had a total of R15 plus free R10 airtime? Now i need to pay the R5 that i did not even use? Tried calling the customer and its not even giving me an option to speak to a consultant. This uis not fair, considaring the fact that we live in tough economic times. Please call me