Airtime dissapear and Vodacom expect me to accept if!!

Had a cellphone contract for each of our family members. All in my husband's name. In September we upgraded some of the contracts. One of them that was on a Topup we didn't upgrade and asked for it to stay as prepaid becausemy son will be using that sim card in his Ipad. A few days later all the airtime that was on the phone dissapeared. We phoned call centre and they told us when you cancel a Topup you forfeit the airtime. I requested to speak to a supervisor but was told that is how it works. Every week we make calls to Vodacom and no one can help in telling where my airtime is. Whenever I ask for a reference for the call I get told it will me sent to the phone Im calling from when Im done with the call and it NEVER happens!. And my husband works offs**** so isn't available until late this year and they want to speak to him because contract was in his name but my husband told them we will be using this sim card as prepaid. All I want is just my airtime bac**** WE PAID FOR IT!!