To verify the authenticity of phone numbers stored in user profiles, developers can use Twilio's Lookup API with plain PHP. The process requires installing three dependencies: Twilio's PHP Helper Library, PHP dotenv, and CLImate. A free Twilio account is also necessary. Once set up, a single file `index.php` contains the code that retrieves phone number details using the Lookup API. The code fetches a specific phone number and prints its country code, national format, international format, carrier name, and type. The output is formatted using CLImate for a polished appearance. The example demonstrates how to verify phone numbers in plain PHP without relying on a framework, making it accessible to developers of all skill levels.