You can now build a passcode protected conference line with Twilio and Ruby on Rails. To do this, you need to install the necessary dependencies, including Ruby and Bundler, a Twilio account, a Twilio phone number that can receive incoming calls, and ngrok for testing webhooks. You then start the server and make a POST request to the existing conference call webhook endpoint to test it working. Next, you add new actions to the controller to welcome the user and ask them for the code, and another action to check the digits against a code and decide whether the caller can join the conference or not. You also need to add routes for the two new actions in the controller. Finally, you test the application by starting it with a PIN in the environment, using ngrok to open a tunnel and give you a public URL, and setting up your Twilio console to use this URL as the incoming call webhook. When you dial your phone number, you will be greeted with your welcome message, enter the incorrect code and you will be denied access, but enter the correct code and you will be dropped into a conference call.