To build a conference line with Twilio and Java, start by setting up a Spring Boot project and adding the Twilio Helper Library to it. Then, create a new class that handles incoming calls and generates a VoiceResponse containing a welcome message and a Dial verb to connect the caller to a conference room. Use ngrok to create a temporary public URL for your localhost app so that Twilio's webhook can call into it. Set up a Twilio phone number and configure it with the ngrok URL, then test out the conference call by calling the number and joining with friends. The project also discusses how to build off of this basic example with additional features such as joining on mute or making calls moderated.