Several phone rental companies have outlets at NRT. It used to be possible for non-residents to buy a cheapo prepaid phone, but that was made illegal early in the 2000s, due to irrational fear of "foreign criminals," as if legal residents couldn't be criminals, too.

If you're not going to be in Japan for very long and don't need a phone for business, consider going without and communicating with the folks back home via e-mail. There's rarely anything that's so urgent that it can't be dealt with by e-mail. (I've never understood why travelers want to call up their family and friends back home and just chat.)

If you have a 3G smartphone, it will roam on Japanese networks and work with Japanese wi-fi, at least in the larger cities, and it will do for a real emergency, although the per-minute charges will be high. Still, it's cheaper than renting a phone unless you're there for a very short time.

Japanese wi-fi is a separate issue, not as easy to find as in Europe or North America, but some of the budget hotel chains have wi-fi in the rooms. The wi-fi in the Comfort Hotels was really strong. Even the Dormy Inn, which had only Ethernet connections in the rooms, had a public computer in the lobby.