AWS Lambda, Amazon API Gateway, and Twilio are used to build an Interactive Voice Response (IVR) system. The IVR allows users to interact with a phone call by selecting options from a menu, which triggers different actions based on the user's input. The system uses AWS Lambda as a serverless compute service to run code that processes the user's input and returns TwiML instructions to Twilio, which then plays back the selected option. The Amazon API Gateway is used to create an endpoint that can be invoked by making an HTTP request, allowing users to interact with the IVR system through a web interface or other means. The system also uses Twilio as a cloud communication platform to handle phone calls and send TwiML instructions to the user's phone. By using these services together, developers can build scalable and flexible IVR systems that can be easily integrated into their applications.