02-24-2019 11:32 PM
Please check your phone app settings. check whether this three are enabled or not. 1. display over other apps(allowed), 2. default phone app(Yes), 3. modify system settings (allowed).
02-25-2019 09:25 AM
Thanks for posting. I know how inconvenient this must be, allow us to help. Can you please try to check if you have an option to change the Incoming calls settings on your device's Settings? Kindly go to Settings > Apps & Notifications > See all XXX app installed > Phone > App notifications > Incoming calls > Importance > Select Urgent (Make sound and pop on screen).
Let us know how it goes.