This tutorial demonstrates how to use Twilio's Programmable Voice and SMS APIs to send a text message during a phone call using C# and the .NET Framework. The code creates a web application that accepts incoming calls, says something to the caller, and sends the caller an SMS with a clue. The application uses Twilio's .NET helper library to interact with the API. To handle exceptions when sending SMS messages to landline numbers, the application includes error handling code.