Single “User-Only” Page with SMS Verification?

I am building a relatively simple site for a client and am hitting a wall when it comes to what seemed like a deceivingly easy request.

He wants one page of the site with important info to be only accessible to users who have provided a valid phone number in a registration form for easy followup later on. I have a couple ideas for how to build that functionality, but neither cover every base.

Thanks for any advice you can send my way. I’m relatively new when it comes to WordPress dev.