This is a form of two-factor authentication — when you sign in to twitter.com, there’s a second check to make sure it’s really you. After you enroll in login verification, you’ll be asked to enter a six-digit code that we send to your phone via SMS each time you sign in to twitter.com.
This is great, and has been an increasingly frequent request over the past couple of years. However, there’s one drawback: You can only use your phone number with one account.
So if you have a public Twitter account, and another one you use for work, or for close friends, or for headlines from your blog…you can only enable two-factor authentication for one of those accounts.
The growing use of phone numbers or email addresses as a substitute for an account name is something that’s been driving me crazy for a long while, this is just another example of how it can unnecessarily hamper users.