original guide here: https://docs.google.com/document/d/1nuQAs1vEsLUoNx6DfoKzxWL61zP-l_mPJ_s8iIRfoYc/mobilebasic
copying and pasting below in case something happens to the above document being hosted
Perhaps you are in a situation like mine where you have a non-Japanese phone without a SIM card tray and you need to get a Japanese phone plan without buying a new phone. Look no further! I got you. I used IIJmio’s Docomo plan, and as such, this guide will show you how to successfully get a Japanese phone number and mobile data using the same plan. This is not necessarily an endorsement of the company, nor is it meant to steer you away from other companies. It’s just the one that I know for sure how to use.
NOTE: For this guide, you will need to have a Residence (Zairyu) Card, Japanese driver’s license, OR a My Number card. The exact specifications of the documents needed can be found here (use your browser to translate the page if needed): https://www.iijmio.jp/hdd/miofone/verify.html. You also need to be at least 18 years old.
You will also, obviously, need a smartphone that is compatible with eSIM. Just in case that wasn’t clear already.
AND A DISCLAIMER: This method is NOT foolproof. I cannot guarantee that it will work for you, unfortunately. However, I can attest to the fact that it is possible
Go here to browse the available Voice eSIM plans: https://www.iijmio.jp/gigaplan/esim/phone/
This webpage does have an English version, but it doesn’t include all of the same information, nor are the other pages on IIJmio’s website in English, so I recommend you use your browser to translate it. Note that on top of the monthly costs, there is an initial fee of 3,300 yen (minus tax), plus an additional issuance fee of 433.4 yen for Docomo or 200 yen for au.
I personally chose the 15GB plan without flat rate calls, because I don’t anticipate needing to make phone calls (as opposed to calls on LINE) except in the case of emergencies. By the way, you can’t make international calls using IIJmio Voice eSIM.