[sirius87]

Curious to hear answers to this too. People forget usernames and that alone would lead to drop off. People are also reluctant to give out phone nos. which is anyway a terrible identifier and has the same issues as email.

[rajasimon]

Why can't the phone number be unique? I have a unique phone number and trust them as my email unless I transfer myself out of my country.

[kahnclusions]

Another edge case is that phone numbers get transferred and reused.

I might verify the number, and then stuff happens in the real world while I haven’t logged into your service, and someone new registers with my number.

The number is assigned to my account, but the new account can prove they own the number. If you’ve made the number unique, then you need special handling here.

[JohnFen]

Just like with email addresses, there are a nontrivial number of people who share the same phone number.