I think I've found the solution - you must swipe DIRECTLY ON the magicJack NOTIFICATION of the incoming call.
Right below the notification it says "slide to answer" in really small letters. (in my case, it was so greyed out it was almost impossible to see). If you swipe anywhere else on the screen, and not ON the notification itself, nothing happens and the call gets lost/disconnected. But, if you literally "swipe the notification" and enter your passcode, that appears to trigger the app to open automatically and answer the call.
Obviously, you must ensure that the notification settings for Magic Jack are ALL "ON" - "Alerts" for alert style, Badge App Icon=ON, Sounds=ON, Show in Notification Center=ON, Include=ON, and (especially) "SHOW ON LOCK SCREEN" = ALL should be "ON"
Hope that helps!