How do I get the alternate phone number to be available on sign in?

Question

I have a media account that needs to be accessed by 3 people. I can use the Microsoft app for one, the primary phone number for the second, but it does not show the alternate phone number as an option for texting the third. How do I get that to be available when logging into the account and selecting the "I can't use Authenticator" option? I'm not seeing any options that affect that, and searching is failing me.