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. You’ll be asked to register a verified phone number and a confirmed email address. Twitter Introduces Two-Factor Authentication To Help Thwart Phishing [blog.twitter.com]
With login verification enabled, your existing applications will continue to work without disruption. If you need to sign in to your Twitter account on other devices or apps, visit your applications page to generate a temporary password to log in and authorize that application.
This release is built on top of Twitter via SMS, so we need to be able to send a text to your phone before you can enroll in login verification (which may not work with some cell phone providers).
This release is built on top of Twitter via SMS, so we need to be able to send a text to your phone before you can enroll in login verification (which may not work with some cell phone providers).
I can't really tell if this is optional or mandatory.
In my neck of the woods, "multi-factor" technically means it goes both ways