What you would have to do is to have the Japanese and Korean phones/numbers available at the same time, from the same place, and then change the phone number. W

What you would have to do is to have the Japanese and Korean phones/numbers available at the same time, from the same place, and then change the phone number. Which is not something that is always possible - re. my post above. As you, I only used the number for the initial registration - and I suspect that what's everybody do. I have no idea what the phone number should be used for outside of that. I'm not sure why Line (well, Naver, the company) does it that way. Others, including Skype (which would need the same level of 'verification') manage it better.