To listen in, browse to the Sessions Tab.

Find an active session in the list and click on the name:

Click on the black arrow icon in the right hand corner of the screen.

You will see a message that you are being connected to Listen in, and when successfully connected, the icon will turn green. 

You will hear the rep as they wait for calls, and will also hear any transfers they are connected to.

When ready to disconnect, simply click the green arrow.