In response to post #33063190. In most cases, two factor authentication requires you to give to service you want to use your phone number. When you try to login, they send you a code that you need to enter directly after inserting username + password. The strength lies in having two independent factors for authentication: something you know (password) and something you physically own (your phone). It can provide more security, but: - people who don't have a mobile are locked out - when they don't send SMS and instead use an app you need to install on your smartphone/tablet to generate these codes, people who own a traditional mobile phone, will be locked out - some people will eventually login on their mobile device which receives the codes, so they might be unlucky enough to defeat the additional protection of 2-factor-authentication for their account While using SMS enables a lot more users to make use of 2-factor-authentication, using an app might be the cheaper solution.