MTN Robbing us of our airtime

I have bought airtime a couple of time. One incident i remember vividly is when i bought R85 airtime and i made one phone call that lasted for at least minute but afterwards my airtime balance was R0.00. I don't know how is this possible... I tried calling 173 customer care line but i had no option to speak to a call centre urgent. Sometimes i do have airtime but when i make a call it will say \you have insufficient funds\". I feel like MTN is *****ing from its customers because i am not the only one who experiences this but my friends too. "