Airtime Disappearing

For the past couple of months my airtime has been disapearing, I am using Blackberry Z10 with only whatsapp installed, no facebook, twitter etc and using prepaid airtime. The airtime will disappear without me making calls and when i check my data balance, its still there. What i picked up is that someone will call me then the line will break for few seconds (maybe 5)and then continue after a while i will get notification that i have less than R3.99 in my account. I found that weird as i did not make the call, its the person i am talking to that made that call. When i try to make a call afterwards, then i have no airtime. This happened a couple of times up until yesterday. I need to buy airtime but i cant cause it might disappear again. Can vodacom sort this out this nonsense and provide me with a detailed report of what happened to my airtime.