The Vonage Number Insight API provides real-time validation of user input to protect apps from fraud and spam, as well as details about phone numbers such as validity, reachability, and roaming status. The API is available in three levels: Basic, Standard, and Advanced, each offering different functionality for formatting numbers, identifying number types, blocking virtual numbers, and determining risk based on numbers. The Advanced API is also available asynchronously and synchronously. To use the API, developers need a Vonage API account and follow a series of step-by-step tutorials to set up an ASP.NET project and integrate the Nexmo C# Client Library.