How can I start a phone card business using VoIP?

WHo will you get to be the VoIP service provider? You would need to set up a deal with them where they will bill you X amount per minute, then you can turn around and make your customer pay X+Y amount.
then you need to get a company to manufacture cards, you would need to have some sort of system set up with the Provider that would acknowledge the card ids... the list goes on.

there are so many choices out there, I might suggest trying a different route. Calling cards are dying out. They are being replaced with pay as you go cellphones... just my opinion. :)