I've been using Cognicall (www.cognicall.com) to make calls from Japan at 39 cents per minutes. It even works from Japan payphones without inserting coins. Cognicall works from most all other Asian countries too. Cognicall does their billing online, charging to a credit card, so if you need paper bills in the postal mail, you'll have to pay extra.
It's been a while since I surveyed calling card rates so I don't know if Cognicall is still the among the best rates...
The cell phone idea sounds good if you're going to be in Japan a while. There's a cell phone rental shop right in the NRT airport mall. Don't know if you'd save alot of money going elsewhere...
[This message has been edited by anthonyanthony (edited 07-28-2003).]