This tutorial outlines how to build an SMS messaging system using Firebase Cloud Functions and the Realtime Database alongside the Vonage SMS API. To start, developers need a Firebase account and a Vonage API account, as well as a virtual phone number from Vonage. They then set up a new Firebase project, install Firebase Tools, and create a function to receive incoming SMS messages via a webhook. The function logs the message into the Real-Time Database, and the vital piece is the Function URL, which is required to set up the webhook in Vonage. Next, developers use a virtual phone number to send a message, log the message into the database, and then create another function to send SMS messages back to the original sender. The new function watches for updates to the Real-Time Database, sends an echo of the original text, and deploys the Firebase Functions again to test the response. Overall, this tutorial provides a comprehensive guide on how to build an SMS messaging system using Firebase Cloud Functions and the Vonage SMS API.