When I lived in Europe, I had a US number on
Skype. I even got to pick the area code so it was a local call for my family. It costs $2.95/month and includes almost unlimited* outgoing and incoming calls to/from US cell phones and landlines. Or you could go with the $12.95/month "
world" plan and communicate with anyone in any country out of the 40 covered by the plan (including Japan). There's even voicemail, and you get an email when someone leaves you a message. Call quality is fantastic as long as your internet connection is good. It was awesome, I recommend it.
*Skype advertises it as unlimited, but there's a fair usage policy. You get a ridiculous amount of minutes that I would imagine it would be very difficult to go over. I don't remember exactly how many. You'd have to read all the details.
Oh and I almost forgot- in case you don't feel like sitting in front of your computer to talk, they make handsets that connect to the computer. I think they make a cell phone too, but they didn't make one for France when I was there. Don't know about Japan.