I hope that this is a more comprehensive explanation,Louise,Qunli Bond
Due to different technologies, mobile phones from your home country may not work in Japan. Most importantly, there is no GSM network in Japan, so GSM-only phones do not work. The following are needed for a handset to work in Japan:

For Voice - The handset must be compatible with a Japanese mobile phone network (typically 3G UMTS 2100 MHz or 3G CDMA2000 800 MHz). Compatible handsets may be used via international roaming (check with your home provider for details) or a rental or prepaid SIM card from a Japanese carrier (unlocked handsets only). Alternatively, phones with wireless network (WIFI) connectivity can use internet based telephone services (voip), such as Skype, when connected to a WIFI network.
For Data - Phones that work in Japan for voice (see above) can also receive and send data (such as e-mails and web content) via international roaming or a rental/prepaid SIM card, but note that the cost for data transfer can easily skyrocket without an appropriate data plan. Alternatively, phones with wireless network (WIFI) connectivity can take advantage of the numerous paid and free wifi hotspots found around the country. See our internet page for more details.
Do Japanese phones work outside of Japan?

Many phones that are sold in Japan can operate on 3G and GSM networks (only in certain countries) with the appropriate international roaming plans, although some of their advanced functions will likely not work overseas. This means that a person with a handset and service provided by a Japanese mobile phone carrier can use their phone when traveling outside of Japan.

Although Japanese carriers will now unlock most phone models, it still may not be possible to use a Japanese phone with a foreign service provider due to network differences. Also, Japanese phones are designed at a hardware and software level to work with a particular carrier. As a result they are not fully interchangeable even between Japanese service providers and some of their features may not function when used off the network they were intended for.