This is something that was not true few years ago. Somehow I got a notion that Japanese phone system is different from the rest of the world and I can not use my Sprint phone in Japan. I have found out now I can, sort of.
When I was in Korea, I turned on my Treo 755p phone function for checking out if the phone works. I was greeted with two text messages, wellcoming me to Korea and also adviced me that it will be 2.29 US$/min for voice call and .001 US$ for 1KB of data. I quickly turned it off. While I was in Narita on Transit, I turned on the phone again and surprised to see that the phone also worked there. 1.99 US$/min and no mention of data plan. In any event, it is expensive solution but having a convenience of being able to carrying the same phone over the world even just for an emergency call is a welcome news.