I have exactly the same issue on OnePlus 5. Haven't been able to find a solution. It's a real problem for me because I rely on vowifi as I live in an area with nearly no cell coverage and I use a sip line for work.
All was fine when using the cell vendors WiFi calling app but they recently stopped support for it as vast majority of phones have WiFi calling built in. After switching to Androids built in WiFi calling (VoWifi) this issue began. Note that the WiFi calling feature is not available by default on OnePlus phones... You need to activate it by going into some developer settings.
If it is of interest this affects most sip clients (not just zoiper). I have tried alternatives and most dont work so seems it is a OnePlus issue.