AIRTIME THAT JUST GETS TAKEN EVEN IF YOU PAID FOR IT............

I have a contract with Vodacom where I pay a fixed amount for Airtime in money value every month as part of my Contract. In June I looked at my balance and found that I have accumulated some Airtime which I was happy about because I was on my way of leaving the country for 6 weeks and I knew I would have quiet a bit of Airtime when I come back. When I arrived back I looked at my Airtime I saw that the Airtime was less than when I left for my 6 weeks. I phoned Vodacom to ask how is this possible and they told me sorry the Airtime expired. Now my question is if I pay for Airtime every month how can they just take it according to me I have paid for it and they just then take my money for free and I get no benefit out of it. So in laymen's terms I want to say they are *****ing my money because I pay for it and because I couldn't use it they just take it. In other words I am paying for something and I am not get anything in return.