MTN not keeping by their contract

Today I suddenly could not make any calls anymore. I have airtime and 500 min voice time left but when I make a call it says I do not have any airtime. Phoned MTN who told me I only get 75 min a month on my Megatalk package, which is incorrect since I pay for 500 minutes a month. But the mtn agent kept saying I only get 75 minutes and any minutes I have left is mtn to mtn. This is nonsense how can my contract be changed overnight. I call a lot and thats why I pay for 500 minutes a month.