You've learned how to trigger a usage alert over a phone call with Node.js using the Twilio API, which allows you to track your usage and set up triggers for specific events. You created a new Node.js project, installed the necessary libraries, configured your Twilio credentials, bought a Twilio phone number, created a TwiML bin, made a call from your application, and finally set up a usage trigger in the Twilio Console that invokes the endpoint to trigger an alert when a certain condition is met.