This article demonstrates how to use WebSocket API with Spring Boot to send SMS messages using Twilio. The application uses STOMP protocol for message routing and SockJS for fallback options in case the browser doesn't support WebSocket. The server-side code includes a configuration class `WebSocketConfig` that enables WebSocket messaging, a service class `SMSService` that sends and receives SMS messages, and a controller class `SMSController` that handles incoming requests and sends responses to connected clients. The client-side code is written in JavaScript using the SockJS and STOMP libraries, which establishes a connection with the server and subscribes to the `/topic/sms` topic for message updates. The application also includes a form to send SMS messages, which is handled by the `SMSController`. After setting up the project, users can test the service by clicking the "Send" button and entering their phone number and message. The server will respond with a callback status message after a simulated delay.