You are strongly encouraged to configure more than one method. What if you only configured the phone app, then bought a new phone and forgot to install the app, or maybe you accidentally left your phone at home? By configuring more than one option, you will ensure that you maintain access to your account, even if there is a problem with your default option.
Log in to FlashLine then select "Settings" then "Update your mobile device information" to configure additional methods for logging in to your account.