Some extra notes in case anyone needs them:
After creating a LINE account, they only allow switching between mobile numbers from certain countries. But that's not a big problem.
In my experience, for non-Japanese residents, there are advantages to registering a LINE account using a foreign number (for P4P), #1 being that you won't have to do the almost impossible age-verification process through your network provider to unlock some LINE functions. One such function is "Add friend by LINE ID", for the life of me idk why it is adult-only.
Why is this useful? Because for 姫予約, some girls forget or choose not to put the QR code, and only list their LINE ID. For foreigner non-residents on the Japanese version of LINE, it would be almost impossible to add the girl's account, but it is 100% possible for a foreign-number LINE to add her via ID search.
For the record, I've tried registering and switching LINE accounts between numbers from 4 countries, incl Japan. On iOS, can't comment on Android. (And if you think your Apple ID is somehow banned on LINE, just make a new Apple ID)