Twilio: 3rd caller can barge in to a call in progress

I have a scenario where a Caller called an Agent. Caller and Agent are already engage. How can a 3rd caller(Supervisor) barge in to listen to their conversation.

Thank You,

Simply dial into the conference using TwiML like this:

<Response>
  <Dial>
    <Conference>conference-room</Conference>
  </Dial>
</Response>

you can use the muted attribute if you want the supervisor to hear the conversation, but not be heard:

muted="true"