The issue that you are experiencing is usually caused by Microsoft Lync. Microsoft Lync is hijacking all HID activity. We cannot prevent this behavior. A workaround to prevent the background noise is to open Zoiper -> Settings -> Preferences -> Audio -> Device sub-tab -> "Disable use generic handsets(HID, Jabra)".
Note that this way the Pickup/Hangup buttons on your headset won't work with Zoiper. Or you can just exit from Microsoft Lync while you need to use Zoiper.