On my 1st gen Moto X, when the phone rings while I’m connected to WiFi, the interface appears, offering the choice to swipe right to accept the call. When I swipe right, the round cursor moves over to the phone choice, and then jumps back to the center and the call does not connect.
The same thing happens with the notification tab. I press the “answer” option, and nothing happens, the call does not connect.
It only recently started doing this in the last couple days. (I’m not sure exactly when because my call history marks those as green arrow completed calls.)