My problem is the same. The instructions in the article do not work for me, maybe because there are no suggested settings for Android 11 on a Samsung phone. My phone is a Samsung Galaxy A01 running Android 11. I AM able to answer if Zoiper is showing, and the calls work fine. Outgoing calls work perfectly. The problem happens when the screen has gone black, or Zoiper is not open and showing. Any help will be greatly appreciated.