Listen - you hear the call, nobody hears you. The call is not touched. Whisper - you coach your agent. The customer does not hear you. Barge - you join the call properly and everyone hears everyone.
Whisper and barge take over the audio of a call that is already happening, so you need to be on a call
yourself first - that is where the audio goes. If the mixer ever stalls, the system drops the monitoring
and puts the call back to normal on its own: the call always outranks the monitoring.
Only extensions on the supervisor list may do this.