Storytime: I bounce between Japan and Korea for work. I was in Seoul for about a year and a half and naturally had a Korean phone number. I used this number to sign up for Line (a popular chat service in the two countries), and I also linked my email address to the account. So when it was time to move back to Japan, I naturally canceled my Korean phone service.
Fast forward 6 months, and my Line account suddenly disappears. I contact customer service, and it seems someone else had registered for Line with my previous phone number -- which was of course release when I canceled my service with the telecom in Korea. I was informed that it's Line's policy to only allow one account per phone number, and thus they deleted my old account when the other person registered. There was no way to recover it. I even reached out to one of my engineering friends that worked for Line, at the time.
Some of my friends, I only knew and communicated with through Line, and I have no way to find or contact them again.
So yeah, fxxk using phone numbers as identification.
This is why I'm not a fan of mobile apps that force you to use your number. Sadly the most popular apps all the cool kids are using (your friends) are the most inconvenient in some regards. I wish Signal would let me use an email instead so I can use it from a tablet, but they insist on keeping my phone number instead. Signal made sense to use with phone numbers when they encrypted SMS but they no longer do messaging without data so it makes no sense to me.
Line is the worse violator of these principles. Last I used it, in order to install it on another device, you had to back up all that device's data (contacts, etc) to a computer, put your account on another device which then triggers a non negotiable wipe of the other device's data whenever it connects to the internet, then download the backup to your new device. Hell.
The first of the two bad thing about Line is that connection to a phone number. Wife got a new iPad, and, because of the other bad thing - that Line only works on a single device - it had to be 'moved' to the new iPad. Which needs a confirmation by phone message. And that phone was in Japan, unlike my wife, and wouldn't have worked for text messages even if she had brought it (phone plan didn't allow international calls or text). Contacted Line and got all kinds of useless advice - at least there are people in the other end - but nothing worked.
Had to create a new account.
The backing-up and restore is the easy part. Not much help if you can't move the account due to that silly phone number lock-in.
All these problems would be solved if they allowed us to use Google voice number. It is a number that's attached to my email and one that I won't discard (assuming I would lose it due to in activity). But no. Even Uber doesn't allow me to use a Google voice number.
Looks like I got lucky, I ran into this where a while after moving a switched to a new phone and couldn't activate line because I didn't have the old number anymore. However I managed to sort it out with the line customer service (even though some of the questions like "when did you first register your line account" I could only guestimate because it was so long ago).
Similar thing happened to me with LINE - didn't have anyone else registering with the same number AFAIK, but simply having lost my session and the registered number meant I lost that account and had to register a new one.
I do have a Japanese number. Perhaps you meant I should have linked my account to my number here?
I'm sure there are things I could have done to prevent it from happening, but I had no reason to believe Line would simply delete my account, as it was linked to my email address. In fact, I only used the number for the initial registration. After that, I only used my email/password combo to log in.
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.
Fast forward 6 months, and my Line account suddenly disappears. I contact customer service, and it seems someone else had registered for Line with my previous phone number -- which was of course release when I canceled my service with the telecom in Korea. I was informed that it's Line's policy to only allow one account per phone number, and thus they deleted my old account when the other person registered. There was no way to recover it. I even reached out to one of my engineering friends that worked for Line, at the time.
Some of my friends, I only knew and communicated with through Line, and I have no way to find or contact them again.
So yeah, fxxk using phone numbers as identification.