I'm running ZoIPer 3.9.0 (library version d0af4c2) on iOS 9.3.2 with "Force Background" enabled (my VoIP provider only supports SIP UDP) and I am encountering significant system-wide audio issues when ZoIPer is running. From searching here, some of these appear to be known issues.
When audio is playing in other apps, the volume is reduced and there is an added low level noise, hiss, and popping. This can often be reduced by pausing playback, switching out of and back to the app, then starting playback again (thus forcing the audio app to the foreground).
When using a headset and dialing, some dial tones will be sent out the built-in speakers instead of the headset, though the call will correctly occur through the headset.
Issue #1 seems to be solely caused by the "Force Background" feature as disabling it resolves the issue, but means I can't receive calls. Is it partially related to how background audio apps in iOS will temporarily reduce playback volume of other audio for notifications (such as GPS turn-by-turn instructions)? Is there hope of resolving this issue?