Airtime disappearing overnight

I bought airtime on a Tuesday 08/09/15 and on Wednesday night before I sleeping I checked my airtime I still had R36.00 and 100 and something of data but Thursday morning when I tried to make a call my airtime was gone, I called MTN on Friday after a colleague told me she experienced the same thing AIRTIME DISSAPEARED I was told that I was using internet at night and it charges both airtime and DATA which is a lie?? My Husband called me this morning telling me his air time of R115. DISAPPEARED last night he noticed that when trying to make a call this morning he was told he had insufficient airtime to make a call ,he called MTN and was told the same story that at 10.05 PM an amount of R115 went off from his airtime because he was on internet again that's a lie he was actually playing soccer that time. I WANT MTN TO TAKE THIS MATTER VERY SERIOUS BECUASE WE ARE GETTING FEDUP SINCE THIS WHERE NOT FIRST INCIDENTS OR ELSE THEY ARE GOING TO LOSE LOYAL CUSTOMERS ABOUT THIS DISAPPEARING OF AIRTIME STORY.THEY NEED TO DO A THROUGH INVESTIGATION AND STOP GIVING US LAME EXCUSES.If you are trying to recover your lost revenue during strike times ??you are going about it the wrong WAY