I have a problem with Zoiper on my Android phone Nexus 5X (Android 6.0.1). If the screen is black because I didn't use the phone for some minutes, in 95% of the cases I cannot receiver phone calls - the phone doesn't vibrate or ring - and in the other 5% it starts to ring more than 10 seconds after I initiated the phone call. On an other device with another software the ringing starts immediately after call initiation. The strange thing is, as soon as I unlock the phone, I see the call and it vibrates. I enabled all mentioned on https://www.zoiper.com/en/support/answer/for/android/18/Making_and_receiving_calls. In the Android settings I also disabled the power save option for Zoiper and I didn't disable any notifications. Is there something more I can do or is this a known issue?