I just bought my first cell phone ever last weekend, for exactly the reasons you describe: my bank (in Japan) stopped letting me log in to my online account without 2FA, which can only be received as SMS. I don't need or want a phone, don't use any of the apps, but I still had to purchase one and pay the monthly subscription in order to access my bank account.
If that happened in North America, you could just use TextNow to create a SMS-capable phone number and use it out of your browser.
I think there is no such thing in Japan: a website where anyone an create a throwaway account and get a phone number in any area code. Telephony is tightly run there. There are some commercial services like that that will give you a phone number, but you have to submit paperwork proving that you're a resident of Japan.
boo