This post explains how to validate phone numbers effectively in ASP.NET Core 3.1 Razor Pages projects using the libphonenumber-csharp open source library, demonstrating its capabilities and usefulness in validating and manipulating phone numbers of all types. It shows how to bind a separate data model class to a Razor PageModel, use the data model to prepopulate data entry fields, return data to the same HTML page used for data entry, and use the ModelState object to return errors from the PhoneNumberUtil object to the user interface using Razor Pages' built-in capabilities. The post provides additional resources for learning more about Razor Pages and ASP.NET Core 3.1 features, as well as information on Twilio's services and AJ Saulsberry's background as a Technical Editor at Twilio.