Call Center Agent Supervisors have the ability to monitor in progress calls.
To do this, go to the Manage Team tab in Cisco Finesse. Select the three dots next to the agent you would like to monitor. Select "Monitor"
The call will then be connected to your agent extension.
If you would like to Barge-In into the call to talk, Select the Barge-In button in the call controls at the top of the Finesse dashboard.