When setting 2FA by SMS, Box should update this part of the web site to specify exactly what format the phone number needs to be entered in.
E.g. "7797123456" and not +447797123456, 447797123456, +44(0)7797123456 etc.

Although it seems to work with or without a leading zero, it would be better to not include it as it is not part of the "phone number"