Android recipient doesnt get calls until he called the caller first

I’m playing around with Jami, because I’m looking for a Videochat tool for all platforms, which can make calls (not meeting in a videochatroom).
I installed the actual jami app from f-droid on my andoid phone (aokpROM android 9) and
the jami-all package from the stable jami Ubuntu repo (20201025.1.5a94f17~dfsg1-1) on my xUbuntu 20.04
Calling from android to Ubuntu works nice. But when I try to call from Ubuntu I get no call on android.
It doesnt matter if the android backgroud service is on or off or if the app is in foreground or not.
But when I call from android to Ubuntu frist, then cancel the call, and the call from Ubuntu to android, I get my call notification.
It works as long as I do not completely shutdown the app (in settings->apps) or until i switch the networks (from wifi to mobile network or vice versa). Then I have to call from android again once, and it will work again.
Can I do something, so that android jami could be called everytime.
It seems, that ubuntu jami cant find the android app in the network, as long as there was no connection from initiated android.
Thank you.

Another case, I experienced was: the call from one android to another works, but calling the same account from an windows-client did not ring.
Now I’m completely confused. So the issue is not on recipients side, but on the callers side?