I am using ZOIPER 3.6.1 Library revision: 35079
The APP is interfering (a lot) with the other APPs. Some of the symptoms of the problem:
a) Waze assistent can't finish a sentence. In the middle of every sentence ZOIPER turns it off
b) RSSRadio stops playing audio
c) When I change the Airplay to the Iphone-Speaker I can hear a low noise. It's like Vinyl End of Record Sound, but in a very low volume. If you are young enough to misunderstand this, you can check the video in Vinyl End of Record Sound
How do I know it's Zoiper? Well, when I close it all this problems stops immediately.
OBS: I use both call options "Keep Alive" and "Force Background".