Vodacom robbing customers with extra airtime

I loaded R120 airtime, after purchasing data I was left with R10 airtime. Because of my recharge vodacom gave me R60 extra airtime which is R1,50 per min to all networks. I made 8 calls lasting a total of 12min44 seconds today. When I check my airtime balance I am left with R1 airtime from the original airtime I bought and the extra airtime has been depleted. This is not the first time this network robs me like this but today I wanted to trace it. I am appauled at how Vodacom rips off its customers. They truly are an expensive network with the most poor service.