A simple Flask application is built to generate, validate and deliver one-time passwords (OTPs) via voice calls or SMS using the Twilio service. The application uses a third-party API called GenerateOTP to handle the actual generation of the OTP code. The application handles user input validation on both the form submission for generating OTPs and for validating OTPs received by the user. It also includes two templates, one for generating OTPs and another for validating OTPs, which are rendered using Flask's Jinja2 templating engine. The application uses Twilio to send SMS notifications and voice calls to users based on their preference. If the generated or validated OTP is successful, a success message is flashed to the user and they can proceed to validate the OTP again. If there are any errors, an error message is flashed to the user and they are redirected back to the generate page.