A third party (Supervisor) can silently monitor a two-party call made by or answered by an 8x8 PBX user (Agent) on a conversation with an external party(Customer). No audible sound would alert the agent or the customer that the conversation is being monitored.
A supervisor silently monitoring a call between an agent and a customer can privately speak to the agent, without the customer hearing the discussion. When the whisper feature is activated, the supervisor can begin speaking to the agent. This is one-directional audio from supervisor to agent. The customer will not hear the whisper. Both supervisor and agent can hear the customer.
A supervisor who is silently monitoring a call decides to enter the conference bridge, joining what then becomes a full three-way conference call, where all three parties can hear each other.