Cell c taking my airtime when i have bundles

I had a 15gig data bundle active for another 6 months. I recharged my airtime, with r100, bought r50 voice minutes. Today when i looked the other r50 was gone out of my cell c app. I phoned the call centre ( on hold for 13 minutes)who said i used it for data, which i did not as i still have 14 gig of a data bundle left. Call centre confirmed this is not right but back office must investigate and i must phone back to find out the reason???? Why must i phone if cell c is taking my airtime and cannot figure out why!!!! Reference for call ***