girafy
You'll find it und er settings > network & Internet > SIM cards > your SIM > in settings menu of your SIM find APN settings at 3rd row from the bottom.
Select your APN profile, find 'APN-type' and open the sub-menue with codes and their check boxes. Look for 'ims' and un-check it if checked, switch flight mode on and off and you hopefully are able to call and reveive calls.