Part of my confusion in replying is you say you got a "japanese" phone. If you bought a cell phone that was made for the Japanese national market, then no, it won't work in the U.S. because the Japanese use a different system than most of the world. (I think it's a variant of CDMA.) On the other hand, if you just bought a japanese-manufactured phone, like a Panasonic, then no problem.

The value of getting an unlocked phone is that you can use it with different providers, if you have their SIM card. However, special functions like Internet access may require some special settings from your service provider.