This tutorial demonstrates how to use the Number Insight API within an ASP.NET Core Web API application, allowing users to specify a phone number and retrieve details like carrier, country, and more. The application provides three levels of detail, from basic to advanced, using Vonage's Number Insight API. A complete source code is available on GitHub for those who want to follow along with the tutorial. To build this project, developers need a Vonage API account, an ASP.NET Core Web API project, and the Vonage library installed via NuGet. The application requires registration of the Vonage client in the `appsettings.json` configuration file and the `Program.cs` file. Once set up, users can test the endpoints using the automatically created OpenAPI/Swagger page. The tutorial concludes with a completed phone number lookup app that provides valuable information about a phone number.