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He's just not in a plane

But then why flight-mode sir?

Maybe in a meeting? When flight mode is on you can't receive calls. Like.. "don't disturb me"
Also when you charge your phone it takes half time for a full charge when flight mode is on.

Unless he has a private jet with wi-fi... :P

But shouldn‘t flightmode also turn of Wlan?

Nope. You can turn it on and off at any time.