Can't convert minutes into airtime

two weeks ago they called and changed my contract. Sweet talking me into believing that MTN are angels and don't ***** anyone. I am trying to convert these minutes into airtime, which, when you google, it says you can convert. As soon as you try: Dial *136*3# and follow the prompts. It tells you to buy, buy, buy. They changed a good contract by fooling me. But everyone that has MTN says that their service sucks, and they don't care what you think about them. My contract started in Oct 1998. Loyalty is obviously for dogs. I know they don't care, much like the ANC, all promises and no action