[hot_gril]

Identifier as in, used for authentication and possibly even tied to your real name. Even messaging apps don't tend to have screen-names like AIM, they have phone numbers, including the so-called "privacy-focused" Signal. Tons of in-person services (govt, banks, etc) will also want one, and it may be used as yet another piece proving your real identity.

[tuwtuwtuwtuw]

Alright. Where I live, phone numbers are public info and not secret so here they are not used when authenticating yourself against a bank or similar.

Using a phone _number_ to authenticate yourself against the government seems completely bizarre to me.

I understand that leakage of phone numbers become a problem if you use them for those kinds of things. But honestly, using them for authentication seems crazy from a security point of view.