Upgraded My Contract without my Consent

I want to know why Vodacom upgraded my contract without my consent , I used to get a certain amount of airtime and sms and if I'M out of airtime couldn't make calls, why was my contract changed to an open line which allows me to make calls even when I don't have airtime? I have never authorized this or requested or be asked, who is eligible to pay for all the bills that have occurred due to this? is this a money making scheme?