SOLVED: Huawei uses a custom power management system. It is necessary to modify settings in order to give Zoiper permission to run in background.
I just installed Zoiper on my wife's Huawei P8 Lite Lollipop. The app seems to shutdown every time the phone is locked. Obviously no calls can be received when this happens. We need to launch the app every time the phone is unlocked. This does not happen on my HTC M8 Lollipop. I have checked the Keepalive options in Zoiper.