Airtime magically "disappears"

This is now three days in a row that I notice airtime disappearing. I load R10, received the R20 mega bonus. The same day, all my airtime is gone. I have two numbers that I recharge at the same time. On the one, which I use most, no problem. On the other one, just disappears. How is that possible. Somewhere someone is *****ing my airtime.