This is a fork of the Request Callback plugin for WordPress by Scott Salisbury, which you can find here. The plugin is what makes the same button on my site here work. Scott’s website is gone, and the plugin hasn’t been updated for five years. I believe in its value and wanted to tweak it for modern installations, especially for my own use.
Most importantly, I wanted the use of shortcodes. This allows me to write “[callback]” wherever I want a callback button. With the latest version of WordPress, I can even do this in a sidebar. Easy-peasy! Before you’d have to edit your template directly. If you updated the template, your changes would get clobbered and you’d have to do it again. Who has time for that?