Airtime consumption

Whenever I buy airtime from Vodacom it gets consumed without me making any calls. I reported this to their call centre the first time it happened in 2014 and I was told to close my app after using them, I took their advice and the problem stopped. To my surprise it happed again beginning of this year, I went to vodacom customer care in grenacres PE for assistance. I asked them what could be the reason cause at first I was told to close my apps of I do evrytime, he checked my phone and keep on guessing by telling me that its because I receive pictures and videos of its not true as its very seldom . He continued to say the reason could be Facebook or twitter of am not using any of them am only using what app and I don't receive a lot of pictures or videos as that's very seldom. The guy was ****ed and told me the only solution is to convert some of my airtime into data bundle or else my airtym will be used. I recently bought an airtym of R50 and bought another of R29 which I converted into data bundles and left R50 for calls, I switched off data mobile and restricted my mobile data, and close all apps and to my shock I made only 2 calls for less than 10 minutes and my airtime was gone