Because of frequency compatibility problems, most U.S. produced cell phones won't work in Japan. 3G or W-CDMA types usually work in Japan but you really should call your provider and check on this before proceeding.
If you'll be a new arrival to Japan for teaching or sight seeing and now know that your phone won't be compatible with the frequencies used in Japan then you have a few options to consider.
Renting a Phone
If you don't want to get tied into a long contract by buying a cell phone in Japan and really only intend on staying a few weeks or a few months then renting a cell phone or buying a SIM card is the probably the best option for you.
Using Your Existing Phone With a SIM Card
If you want to use your existing phone with a SIM card then you have a few options. You can buy a SIM Card (outside of Japan) and put it in your own GSM compatible phone. (Basically the SIM card will let you hook you into a compatible network. This also allows cheaper rates. And you'll be issued a new phone number.) Rates are not bad especially if you call off peak hours. When returning the phone you can drop it in the mail with self-addressed envelopes or at the airport for most companies.
Japan Cell Phones: Renting in Japan Can Be a Hassle
If you choose to rent a phone in Japan, you'll have to bring your gaijin card, passport and also have to deal with rental agents who sometimes don't speak English. Sometimes, even a bank account is necessary. That's why The smartest route is to rent one before going to Japan.
Pre-paid SIM Cards Not Available in Japan
Right off the bat we need to say that if you are intending on using your existing cell-phone and then pick up a SIM card in Japan so that you can use your phone over there, that won't work as pre-paid phones are available in Japan but pre-paid SIM cards aren't. You can however buy SIM cards before going over to Japan. One thing to keep in mind is that if you'll be traveling to 2 or 3 countries you'll need a multi-country SIM card. But if you intend on only visiting Japan, then a single country SIM card or country specific SIM card will save you some money as you can take advantage of cheaper local rates.
Japan Cell Phones:Other Options
Four, and we don't recommend it because of how high the rates are but you can simply buy an international calling plan or roaming plan. You can keep your phone and number but it's all for a price. If you guessed you'll be charged an arm and a leg per second, you guessed right. More info. on getting cellphone service in Japan here.
Lastly the cheapest of all workarounds is to use your computer and Skype to handle your communication needs. The only problem with this is that you'll be tied to your computer and finding an internet cafe and then paying the cafe their rental fee adds up pretty quickly. But if you have internet access at your hotel, youth hostel or where ever you're staying and have or have access to a compute this will definitely save you some money.