Ciao binobany. Thank you very much for your reply. Unfortunately it did not help. Same fault still pops up, when doing a call - I cannot operate the app. I have attached a Screenshot (although it is in German writing, sorry)
Brother, Verify that the SIM card is correctly installed and not damaged. Contact the carrier for assistance testing the SIM or for a replacement. If you have another active phone on the same network, you may be able to exchange SIM cards between the phones to test them.
Wipe your cache partition. (Switch off your phone. Press and hold, power button, home button and volume-up button for 4-5 seconds till the phone turn on and boot into the recovery mode. Use volume keys to navigate to 'wipe cache partition.' Use power key to select and confirm. Then reboot your device. Normally, this will not effect any personal data or settings.)