i think we have to start with a other Point.
I use the Phone since Friday with no Problems by making some longer Calls (1 Hour) and my phone rings everytime so no forwarding to the Voicebox. -> Phonecalls are working
So i think the problem is to search in the type of Networks.
i am located in Germany (Munich) and use Vodafone GSM/WCDMA/LTE
With the Data Connection i have some Problems in the underground but this i have with every Phone. I have now made the change described here, And will test it the next days.