It depends on the carrier you want to use. Historically the carrier that handled international calls was KDDI. They tend to be the most expensive. Here's how to use a credit card with them:

http://www.kddi.com/english/telephon...apan/card.html

You can buy pricey prepaid cards at Narita as well which will still be cheaper than the above method. The good discount cards tend not to be for sale at Narita, I don't think.

Whoops, too late, the OP is probably gone already!