I'm using a LG V20 phone with Zoiper Pro.
It used to work, and I haven't used Zoiper for a few months. When I tried to use it again starting yesterday, I could make calls, however when an incoming call comes, it rings without the ability to answer the call.
(I am able to answer if using a Bluetooth headset via pressing the headset button to answer).
Basically, the problem is the "slide to answer" doesn't appear when an incoming call comes in.