This guide demonstrates how to build a call scheduling service using Twilio Programmable Voice and the NestJS framework. The application is built with a modular architecture, consisting of Controllers, Services, and Modules, which supports scalability, maintainability, and microservices readiness. The project uses the Nest CLI to scaffold the application and integrates Twilio APIs for scheduling voice calls while ensuring quality through unit tests. The application also includes a reminders resource to handle scheduling logic, which is implemented using cron jobs and the SchedulerRegistry. The guide covers key aspects of building the service, including integrating Twilio, creating unit tests, implementing services, and testing the application.