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I recommend OCN.
https://506506.ntt.com/english/ipphone/plus/charge.html
Extremely affordable. With the data only plan + 050 number, you can call almost any number, except toll free and 119/110. Use apps to bring up local emergency numbers quickly as a replacement.
Local police station numbers: https://play.google.com/store/apps/details?id=com.astonworks.emergencynavi
Local hospital numbers: https://play.google.com/store/apps/details?id=jp.co.agoop.hospitalchecker
There was an app in beta I was using for fire stations, but I can't find it at the moment.
You can also use Google Maps to get phone numbers for local emergency locations.
My husband and I started out on this plan, but we have a contract with OCN now for cell tower voice and text for the added bonus of calling toll free numbers, emergency numbers, being able to use text verification, and being able to turn off data, but still receive calls to take a break from the internet.
I'm on a spouse visa and he's a citizen of Japan, so I don't know if the contract plan's available for someone coming over on a work visa. But the contract-free data-only 050 number plan should be fine.
Added bonus, unlike it was for us in America, there are no real data limits. You don't get charged for going over, it only gets slowed down. So you can use as much data as you need. I recommend using apps like Line or Skype with friends, and just saving the 050+ number for making important calls.
Compatible devices: https://506506.ntt.com/english/ipphone/plus/notice.html
Although, recently my Huawei broke, and I got a cheap Doogee Galicia, and the 050+ app works fine on it, and that phone's not on the list. So, I imagine it works with most unlocked phones as long as they have a sim card slot.
If your phone has a sim card slot, it will probably work. If not, consider the Huawei Stream S. Extremely affordable for around 13000 yen. It has a really nice camera too. I miss the camera. (There's also the Doogee Galacia for around 9000 yen... even more affordable.)