Airtime running out without using phone data

How is it possible that you loose your airtime but you never made a call or switched on your Roaming or use data? Where ever I go I just use Wi-Fi at work or at home. This morning I then received a text that "Your balance is lower than R5.00. How if I never called or even switched on my mobile data that remains off?

Please explain as to where this airtime disappears or do we have a case that can be escalated to court for theft?