Yes, this isn't a Problem at all. Just during the time my phone is switched off and in fly-mode the problems occur…
I can't imagine why!?

Problem still occures... anybody, who can help me?
Just for better understanding:
When Jolla-Phone is switched on and not in fly mode messages will arrive on my phone.
When it is switched off or in fly-mode, messages of iPhone-user don't come through.

Update: One of my Friends (iPhone-user) can't write me at all via whatsapp. Maybe they (iPhone-Users) have to change somthings in settings!?