CHARGED FOR INCOMING CALLS

I have a cell c contract. I was made aware by friend that they could not reach me because my phone is always engaged. I discovered I have run out of airtime. I recharged my cell account with R 44.00 and asked my wife to phone me. We spoke less than a minute. I checked my airtime balance again after the phone call and discovered I have charged R 3.60 for the call my wife made to me. ? Why can I not received any calls If i dont have airtime available and why did I have to pay for the call my wife made to me. I did not make the a call to her. This is daylight robery.