I found a similar request for Skype here (https://www.skypefeedback.com/forums/299913-generally-available/suggestions/8900035-monitoring-features-silent-monitor-whisper-bar). I haven't been able to find documentation on implementation of the following so wanted to throw it out there just in case.

1) Whisper - Managers or supervisors can tell agents what to say (advice or answers) without customer hearing them.

2) Barge In - Managers or other users can jump on a call without permission from the original user to help with a call (especially useful in call center situations).

3) Listen - Managers or other users can listen in on a call, given they have the access (QA uses extensively).

All 3 features are sometimes referred to with different names, but hopefully the concept is clear.