I am travelling to Japan and I don't see a reason why calling over WIFI (which my phone supports) should not work.?
So I am carrying a Japanese MIFI so I think I should be able to send/receive calls?
I've also added Skype just in case.
BTW, are there any Japanese test numbers I can call? In the US, there used be vocal time phone number (popcorn) that spoke the current time (US) but I'd like a Japanese number that I can test.