From the navigation menu, tap Settings and privacy.
Tap Phone Number and verify your password. If you haven’t previously added a phone number, tap Add.
Note: If you have already added a phone number, tapping this button will give you an option to Update number or Delete number. Learn more about updating the phone number on your account.
Enter your phone number in the empty field and tap Next.
Twitter will then send an SMS text message with a verification code to the phone number you added.
Open the Twitter app again and enter the verification code. Tap Verify.