I am running Zoiper 3.1.12 (library v2.8.26-IPv6) on iOS 10.2 (though this occurred under 10.1.x as well) and several months ago it began crashing after registering on the initial launch. Sometimes it'll crash on subsequent launches, but usually the second launch (when it still shows as a running app) will work normally.
It launches and loads fine, and registers my UDP SIP provider successfully (starting with unregistered, then registering, then registered), and shows "WiFi is Available" notification, but quits back to the home screen when the "WiFi is Available" notification completes animating off screen. Upon re-opening the app it goes through the registration process again (successfully) and then doesn't crash.
As my SIP provider is UDP-only and I want to receive calls, I have Zoiper set to "force background" and "WiFi keep alive". Once successfully launched, it continues to operate normally.
This is quite frustrating. Unfortunately, I am iOS only at this time (no Mac/PC to connect the iOS device to), so I haven't been able to download any debug logs to iTunes.