Same feature family, three quite different levels of interference, and one of them has consent implications.
Monitor means listening without either party knowing. Whisper means talking to your own agent while the customer cannot hear. Barge means joining the call so that everybody can hear you.
Whisper is the useful one day to day. A new person is on a call, gets stuck, and somebody senior gives them the sentence without the customer hearing a handover.
Barge is for rescue. Monitor is for coaching after the fact, and for quality checks that would be pointless if the agent could hear you.
In several places a supervisor silently joining a call has the same status as recording it, and the same disclosure requirements. Whether your announcement covers monitoring as well as recording is worth checking rather than assuming.
We are not lawyers and this is not advice — but the question is cheap to ask and expensive to skip.
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