This is an extremely minimal web form written in PHP that people can use to provide their phone numbers. The numbers are stored in a text file called numbers in the same directory. This is intended to be used with a shared hosting service, but self-hosting is straightforward if you have a web server with PHP installed. In either case, just drop it into a folder on your web server and it should work.
This repository includes a .htaccess file which denies public access to the numbers file. This only works on Apache servers which have .htaccess enabled. Most shared hosting providers enable this feature. If you are using a different web server, you will need to find a different way to protect the numbers file. If you want the numbers to be public, you can remove the .htaccess file. You may also prefer to use basic web authentication instead of denying all access to the file.