- Log in to your account.
- Visit your Account Settings page.
- Select Require a verification code when I sign in.
- Add you phone number.
- After the process is complete you would be prompted to enter a 6-digit code during each sign-in which would be sent via SMS to your phone.
Twitter Product Security Team adds that existing twitter apps wouldn't be affected by the move.However, new apps or devices would need authorization by generating a temporary password on Applications page.