A developer is building an Interactive Voice Response (IVR) system using Twilio and Django. The system will use a Twilio phone number to route callers through different options, with the response determined by the digits entered by the caller. The IVR system will be built using the django-twilio library, which simplifies the process of integrating Twilio with Django. To build the system, the developer will need to create a new Django project, install the django-twilio library, and configure the Twilio phone number to route calls to a specific URL. The developer will then write code to handle the incoming requests from Twilio, using TwiML (Twilio Markup Language) to generate responses based on the user's input. The system will be tested by making a call to the Twilio phone number and entering different digits to see how the response is generated.