You've set up a usage trigger with Twilio that sends an SMS notification when your account reaches a certain threshold. To trigger this alert, you made a voice call using the Twilio CLI, which sent an HTTP request to your Express server app. The server received the request and processed it by sending an SMS notification to your personal phone number. You can customize the trigger settings, such as the callback URL, trigger value, and usage category, to suit your needs. With this setup, you can monitor your Twilio API usage and receive alerts when you reach specific thresholds.