In the registration page, you will need to modify the form to make the phone number the username.
Disable "Visitor Name" and "Accept Terms" if you are not requiring the terms and conditions. Edit the "Email" field.
Change the Label, Description, Validation, and Validation Error.
Then edit the login page, and set the authentication to "Access Code"
That should allow you to use the phone number as username and no password for login.