I'm not very keen on those two-factor approaches like this that use text messages to your phone. The text message often does not arrive. And in some places you have to pay to receive text messages so it can cost money to log in.
It would be preferable if they could just use the Google Authenticator app like a few other sites do.
There is a 2-factor code generator built into the Facebook app for iOS and Android specifically to help with SMS deliverability problems. Have a look at https://www.facebook.com/help/270942386330392/
It would be preferable if they could just use the Google Authenticator app like a few other sites do.