This feature allows you to intrude on another user and be heard by them without being able to hear the user's existing call which is not interrupted.
For example: User A is on a call with user B. When user C intrudes on user A, they can be heard by user A but not by user B who can still hear user A. Whisper page can be used to talk to a user who has enabled private call.
The system support a range of other call intrusion methods in addition to this feature.