This may not apply to the OP but if you fly to/from Japan on certain airlines a Japanese cell phone can be rented free of charge (you only pay for the calls and optional insurance). I have rented a cell phone twice in the last 6 months from TelecomSquare by flying to/from Japan on ANA. JAL may also offer a similar arrangement but I can't find any information in English.

I regularly rent a pocket wi-fi from Rentafone Japan who also have voice-only* cell phones for rental.

* I don't think there are any Japanese cell phones which are for voice calls only but you don't have to use additional features such as email.