Use Firebase in Web App via Phone Number Log in to Restrict Access to Site?

Firebase now provides an authentication method using the users phone. A user opens a web page, and the page initiates a SMS verification process. Firebase stores the users phone number (Identifier) and a randomly generated User UID. This is great if you want to validate phone numbers of your users, but not so great for restricting access to a whitelist of known phone numbers. Using Firebase phone auth and the very handy firebaseui-web library you receive an authorization when a phone number (ANY phone number) is validated via SMS message.

I guess I could perform a test in the browser client to see if the phone number (after SMS validation) is on the authorized whitelist via an API call to another server; If valid team member, redirect to the team directory site and to a "thanks anyway" site if not. I will say, performing that query in the client browser JavaScript seems like the wrong thing to do. It would be too easy to cheat it. Definitely not a good thing to do,

I'm hoping to use Firebase and phone SMS authorization to restrict a web page's access to a white list of authorized phone numbers. My use case is a group of people who need access to a simple web page showing the phone numbers of others in the group. We don't want that visible to the world, but it would be nice to have in a handy form (i.e. Look up web page, press the <a href="tel:1-408-555-5555">1-408-555-5555</a> phone number for quick, easy access on a mobile phone. We intend to make the page persistent via Progressive Web Application methods.

Any hints on how to proceed on using Firebase to restrict a webpage to a whitelist of users' phone numbers?

Note: I've seen this response that hints that it may be possible to use Cloud Function to restrict access to content but its not clear to me how that would work.