SMS is not working for a co-admin. Issue has been going on for days.

Our co-admin can access box via the box app on their mac but cannot access the web app as they are not receiving sms from box.com. This is only an issue with box.  It works fine with MS and Adobe.  How can I switch this person to use the an authentication app? Please don't ask them to login to their account, as they can't do it because of the sms issue.