You can send SMS messages by creating a Next.js server action that uses the Vonage Node.js SDK. This action handles sending messages securely on the server-side and interacts with a form component on the front-end. The form collects the recipient's phone number and the message text, which are then sent to the Vonage API via the SDK.

A Vonage Delivery Receipt (DLR) is a status update sent via webhook to your application, confirming the delivery status of an SMS message. It provides information on whether the message was successfully delivered, failed, or expired. The DLR is crucial for tracking message delivery beyond the initial 'accepted' status.

Inbound SMS messages are handled using a webhook route handler in your Next.js application. When someone sends a message to your Vonage number, Vonage forwards it to this endpoint as a POST request. The handler processes the message content and sender number, allowing your application to respond or trigger actions.

Vonage uses webhooks for delivery receipts because message delivery is an asynchronous process. The initial API response only confirms message acceptance, not final delivery. Webhooks provide real-time status updates as they become available without requiring your application to constantly poll the API.

Zod is beneficial for validating user input in your Next.js SMS application. By defining schemas, you ensure data integrity and prevent invalid phone numbers or message text from being processed. This helps avoid unexpected errors and improves the reliability of your application.

Prerequisites include a Vonage API account, a rented Vonage virtual number with SMS capabilities, Node.js and npm or yarn, Git, and a deployment platform account (like Vercel). Basic understanding of JavaScript, React, Next.js, and terminal commands is also recommended.

Create a .env.local file in the root of your Next.js project and add your VONAGE_API_KEY, VONAGE_API_SECRET, and VONAGE_VIRTUAL_NUMBER. This file should be added to your .gitignore to prevent sensitive information from being committed to version control. On platforms like Vercel, set environment variables within the project's settings.

The guide uses the latest version of Next.js with the App Router, which provides improved routing and server-side capabilities. This simplifies the process of setting up API routes and handling webhook requests.

Testing webhooks locally requires tools like ngrok to create a public tunnel to your local server, as Vonage needs a publicly accessible URL. The guide emphasizes deployment for proper webhook functionality due to this requirement.

Configure webhook URLs in your Vonage API Dashboard settings. Provide the full public URLs of your deployed Next.js route handlers for delivery receipts (DLRs) and inbound messages, ensuring the HTTP method is set to POST. These URLs are where Vonage will send webhook data.

To troubleshoot webhook issues, double-check URL accuracy in the Vonage dashboard, verify successful deployment, ensure your handler returns a 200 OK response, and check firewalls if self-hosting. Inspect your Vercel function logs for errors, and ensure correct Vonage number format and SMS capabilities.

Key security measures include using environment variables for API credentials, implementing thorough input validation with tools like Zod, and potentially incorporating secret tokens or rate limiting for webhooks to prevent misuse or unauthorized access.

A complete, working code repository can be found on GitHub at: https://github.com/vonage-community/vonage-nextjs-sms-guide. This allows you to explore a fully functional implementation of the concepts discussed in the guide.