How to program prepaid cards to make random numbers that will be use as PIN number?

I bought "minutes"from a 900 service. I would like to package them into "calling cards"





I want to know how to get the application program that makes random number so that whrn the customer scrach the card and type the random number on the phone, the computer accept it and count the minutes down as they are used.

How to program prepaid cards to make random numbers that will be use as PIN number?
I think you might need to explain a little bit more clearly what exactly it is you are trying to accomplish.