Kojichu

It would be so nice to come on a business trip to Japan, switch on your cell phone from your home country and start communicating. But unfortunately only few phones can connect to networks in Japan.
Which phones work, which don't?

2G phones will definitely not be compatible between Japan and other countries as Japan's PDC (Personal Digital Cellular) is a TDMA based technology only employed in Japan. GSM does not exist in Japan.
For 3G networks, Japan's carriers NTT docomo and Softbank have employed UMTS.
If you have such a UMTS phone, your chance, that it will work in Japan is high. Please confirm before your trip to Japan.

In case you have a compatible phone, you can communicate as if in your home country with your number! To dial within Japan, you dial the local numbers and you will be charged "roaming" rates, which are very expensive compared to your local charges at home. Plus anybody, who calls you (except from within your home country), will be charged the call rates to your home country and you will have to pay for the portion of this call from your home cell to where you are at the moment.

If your phone will not work in Japan, or you don't want to pay roaming charges, you have some options:

Rental phones
Rental cell phones are available in Narita Airport from dozens of companies www.narita-airport.jp/en/guide/service/list/svc_19.html , or via web site pre booking from companies like www.gomobile.co.jp. Or you can ask your hotel, if they can provide you with a rental cell phone - a lot of the better hotels will offer that service. The handsets can usually more or less be switched to English menus. The rates are usually a bit higher, than the normal local rates, but less than roaming rates. Incoming calls are free.

Prepaid phones
If you are not a registered resident of Japan, you will not be able to get a prepaid cell phone.
But if you know somebody living in Japan, who would be willing to make a contract for a prepaid cell phone on your behalf, that would work. The handsets can usually more or less be switched to English menus. The rates are usually a bit higher, than the normal local rates, but less than roaming rates. Incoming calls are free and for outgoing calls you have to constantly "load" your credit to be able to call out. That is simple though, as you can buy credits at most convenience stores 24/7.

Rental SIM Cards
Only for those who have a compatible UMTS handset and who want to avoid the expensive roaming charges. You get a Japanese number with this rental SIM card, that will cost 105Yen per day and you can phone inside Japan for 105 Yen per Minute. Available from Softbank.

Note: The technology evolves pretty rapidly. This article shows the state of July 2009.
Other interesting information on cell phones in Japan can be found here:
euc.jp/misc/cellphones.en.html
ewww.japan-zone.com/new/mobile_sim2.shtml
ewww.japan-zone.com/new/mobile_sim2.shtml