Traditional calling cards for home phones were either completely random or chosen by the customer. There was no algorithm involved. Newer prepaid calling cards might have an algorithm that could be figured out, but you'd run into tons of unactivated ones since stores carry them, but they're only activated after they're paid for. It'd probably be really hard to find a working card since people just don't use them much anymore. Not worth the effort when you can just buy unlimited long distance on Skype for just a few bucks a month.
