Actual handset testing
At some point you will need to test features etc on real handsets. All big 3 carriers have test centers, but they are reserved for on-portal site operators (confirmation missing)
Other testing facilities seem to be fairly costly. (A 5 hour/10 handset session adds up to about 30000 - 42000 jpy)
In addition to these handset labs, there are also places you can rent handsets, which makes more sense if you want to test a handset over a longer time:
DIO Mobile - about 20,000 JPY for one handset with pakehoudai for two weeks.
In addition, mobalean provides quality assurance and handset testing services.