I've turned on "Keep running after screen off" under battery settings, but after leaving the Zoiper screen for a different app, or turning the screen off, chances are good that it will become unable to receive calls.
The Zoiper notification will still be running, but the number doesn't ring and goes straight to voicemail. Then when I relaunch Zoiper, it appears to be just starting up, showing the splash screen.
I've tested the latest Android beta as well as standard versions on my Honor 8, and both exhibit this problem.