Rakuten Saiyo Plan Requires Phone Number For A…. Phone Number?

I’m currently studying abroad in the general Tokyo region for about a year. My school requires that I get a phone plan that isn’t data only, so they need a phone number.

My phone isn’t e-sim compatible, and I don’t use a lot of data, so I was looking at the IIJmio giga plan and the Rakuten Saiyo plan.

The IIJmio giga plan takes a very long time to ship the physical sim and the Rakuten Saiyo plan requires a phone number for sign up. I’ve tried putting my schools number and my dorm’s number, but they say that the number must begin with 080 or 090.

What are my options here for a cheap phone plan? I never plan to use the number, I just need to have it, so being charged by the minute is fine.

I’m trying to avoid Sakura and Mobal since they tend to be significantly more expensive due to being targeted at foreigners. I also have a Visa card I can use for charges and a residence card already.

Edit: Iijmio also requires the contact number. I’m signing up for a phone number, why do I already need to have one?