Vodacom airtime theft??

Vodacom are a complete rip-off when it comes to prepaid airtime. Whenever I buy a month’s (I check the expiry date) airtime and don’t use as I don’t need to, within a week or so when I do want to make a call, I find the airtime is gone. This is nothing less than theft! Why do Vodacom ***** your airtime? It is nothing more than abject theft. I demand a response from them about airtime theft!!