If a call is currently in progress, it will be visible on the Call Monitoring page and will have a Monitor button. To listen to a call, click the Monitor button on the call you want to hear.
A popup window for the call will appear, and you'll automatically be silently listening in on the call. The sales rep and customer on the line are not notified that you are on the call.
If you want to Barge and actually talk on the call, click the Barge button. The footer of the window will now say Mode: Barge. The call turns into a full three-way conference call where all three parties can hear each other. This way, the manager can chime in on the call if needed.
You can easily switch between Listen and Barge by clicking the corresponding button.
Click Leave, or the x in the upper right corner to close the window and stop interacting with the call.