Then don't use so called 'secure' services that rely on phone numbers. There exist plenty of secure chat apps that don't have this flaw. LINE is probably my favorite.
Fair, but it's E2E by default, and only requires a (burner) email address to sign up. If you're careful, there's not much to be gleaned from the remaining data.
Doesn't LINE require a phone number or a Facebook account? Seeing as Facebook will often ask for a photo of you to verify your account before you can use it now, this doesn't seem like a fix for the anonymity issue.
You can sign up with a phone number or email address.
*Untrue. See comments below.
How? As far as I am aware, you can link an email to an account once it is created, though it doesn't really do much, but you require a phone number to create one. Indeed their website claims you need a phone number or Facebook account. I just tried registering using the Chrome extension and it required a phone number.
It appears you are correct. Now I wonder if that's changed recently or age is catching up with me. I've not used it in over a year, but what you said is definitely the case now.