“The bottom line is society needs identifiers,” says Jeremy Grant, coordinator of the Better Identity Coalition, an industry collaboration that includes Visa, Bank of America, Aetna, and Symantec. “We just have to make sure that knowledge of an identifier can’t be used to somehow take over the authenticator. And a phone number is only an identifier; in most cases, it’s public.”
Think of your usernames and passwords. The former are generally public knowledge; it’s how people know who you are. But you keep the latter guarded, because it’s how you prove who you are.
The use of phone numbers as both lock and key has led to the rise, in recent years, of so-called SIM swapping attacks, in which an attacker steals your phone number. When you add two-factor authentication to an account and receive your codes through SMS texts, they go to the attacker instead, along with any calls and texts intended for the victim. Sometimes attackers even use inside sources at carriers who will transfer numbers for them.