If you use B-mobile (from my experience few years back), you'll either need a Japanese resident's home phone line to activate your sim card (so they can track you). Hotels won't do it for you.
Either that or you need to pay a certain amount, I think it was like ~3500yen extra to waive that process. I actually do have a family member living in Japan, so I haven't explored into whether there are extra roadblocks regarding the waiving process.
That's why there are sim card rental/cellphone rental + sim card) businesses in Japan - cause they know foreigners don't get easy access to working sim cards.
Phone rentals are at the airport if you want that option. I don't know if the airport has a sim card rental stall, but I'd look into that option first if I were to go to Japan without a family member living there. If not, then look into some japanese sim card rental websites and see if somehow they'll send it to the airport for pick up.
Don' be fooled by Japan's supposedly free wifi everywhere voucher. They don't work anywhere. Not even at train stations. Once you're out of the airport, you're disconnected in a country where barely anyone speaks English.