Prepaid Phone Card Pin Number Generator

I currently sell prepaid phone cards in The Bahamas. I've been looking online for a Prepaid Phone Card Pin Number Generator but havent been able to find one. The phone cards use a total of 12 numbers in sets of four. For example a card pin number maybe 2933 2304 6056. I know the pin numbers are suppose to be random, but i'm sure it cant be completely random. There must be some kind of code to help generate the numbers. I was wondering if any one out there has a Prepaid Phone Card Pin Number Generator or can develop one?