Yes the target receives the whisper notification, but the audio does not arrive, it can only hear if the target customer is seeing the customer who sends the whisper.
This can be easy to test, just create two records put one on Channel A with subscription 75 and another on Channel B a client without being able to see the client from Channel A. The client from Channel B will receive the whisper notification, but the audio comes nothing.
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I just tested it, the notification doesn't arrive either, but if the customer enters the same channel and sees the customer sending the whisper, he can hear the whisper.
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So if he now leaves channel A and goes back to channel B, stop seeing the customer, he can now hear the whisper, even without seeing him.
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Now, if I disconnect from the server and go back to channel B without seeing the client on channel A, I stop listening to the client's whisper on channel A.