In a faqs page on zoiper support about troubles receiving incoming calls, it recommends:
Open Zoiper -> go to settings -> Connectivity
Enable Keep Alive WiFi (unless you want to switch to 3g/4g when the wireless turns off).
Stay awake is on
Run in background is on
I have enabled "keep alive wifi" and "run in background" but there is no option for "stay awake". However there is an option "background mode" which I have set to "use constant wake lock". Is this the same thing as "stay awake"?