No phone calls possible a day after flashing eOS on FP3+

After flashing eOS on my Fairphone3+, everything worked perfectly as far as I could test it. (camera, telephone, Internet, App-shop…) Next day I could not make or receive any phone call.
Mobile data is still working. If I call it stays in status "connecting"and after a minute it stops. If someone calls he gets the information that I am not available although my phone pretends to be connected to my provider. I have to mention, that we had the same issue with lineage OS on a fairphone 2.

Why could I get and put phone calls for one day? Why do I have still mobile data but no phone connection? Did any body else flash eOS on a FP3

Regain your privacy! Adopt /e/ the unGoogled mobile OS and online servicesphone

Possibly solved! I just deactivated calls in the Threema app (Threema is an alternativ messenger)
Currently it works. hopefully it is only a bug in the Threema app