I have an issue with incoming Zoiper calls using SIP on WiFi on different devices (for example Samsung Galaxy A3).
When the screen is off, about one tenth of the incoming calls receive the busy signal. Often, the moment when the calling party receives the busy signal, the phone starts ringing but cannot be picked up.
Have had this happen on old and newest Zoiper versions, Android 6 and 7.
Changing into TCP or UDP does not make a difference either.
Zoiper is removed from the "optimize battery" list and Android Doze is disabled.
If I keep the phone screen turned on all the time, this does not happen. It only happens when the screen is off.
It took a while until I was able to pinpoint the issue - would greatly appreciate some help!