A friend of mine is over in Germany with her husband, and has been for a couple years now. She uses an internet phone company to have a telephone number in the united states that rings on her cell phone --- in Germany. She has it local to her family, so when they call each other, it's just like a normal phone call... that is to say, it's free.
I can even call her, though it's long distance to her home state... which is to say it's also free, because I have free LD.
If you like that option, I can get specifics from her on how to set it up. I think she pays like $15 or less a month to have the phone line set up, PLUS it's all handled stateside, so your age isn't a big deal... at the very least I can get you the name of the company she does it through and they could answer questions specific to setting it up and using it in Japan.