A few days ago, JAL discontinued email verification codes and now requires a One Time Password sent to your phone number. But the phone number I have on file is a Japanese number, and I can't receive it outside Japan.
Does anyone know a workaround? Ideally, I'd like to change my phone number to a non-Japanese one.
Start by checking the personal information page in your JAL account to see whether the phone number field can be edited directly. Some airlines only allow you to change your contact number after logging in, but before making the change they often require verification via the old number—so the prerequisite is that you can still receive a one-time code on your original Japanese number. If you can't receive it at all, the only option is to go through manual identity verification with customer service; don't keep trying on your own and end up locking your account.
Looked at another way, this really comes down to the airline narrowing verification to a single channel, which isn't very friendly to people who live abroad long term. Whether you can switch to a non-Japanese number depends mainly on whether the system restricts by registered region, not just on whether the page lets you enter one.
Have you confirmed what materials customer service needs to verify identity?
Can your Japanese number still receive text messages right now? If you're just outside Japan, maybe first check whether international roaming is enabled?