** Building a conference line using Twilio and Ruby on Rails is a straightforward process that requires some basic setup, a few tools, and a bit of code. The process starts with installing the necessary gems, including `twilio-ruby`, which provides the functionality for interacting with the Twilio API. A new Rails application is then created, and a controller is generated to handle incoming webhooks from Twilio. This controller uses the `twilio-ruby` gem to build up a TwiML response that instructs Twilio to drop callers into a conference call. The process also involves setting up a ngrok tunnel to expose the local development server to the internet, allowing it to receive webhook requests from Twilio. With these tools and some basic code, developers can create their own conference lines using Twilio and Ruby on Rails.