SMS Passwordless Authentication

Absolutely DO NOT use SMS authentication. It isn't secure[1][2][3] for two-factor auth, and it's definitely not secure as a single factor.

1. https://www.wired.com/2016/06/hey-stop-using-texts-two-facto...

2. http://www.cybersecuritytrend.com/topics/cyber-security/arti...

3. https://www.theregister.co.uk/2016/12/06/2fa_missed_warning/