Alas, some apps will stop your device from ringing. This isn't a setting in the phone, but one within the app that is blocking it from ringing. So you are going to have to determine what app that you have running is causing this (the phone to not ring but give you a pop up) and then check the settings in that app to see if you can change the option to allow it to ring.
Now if that app doesn't have the option to change it to allow your device to ring, then you would either have to stop using said app, or contact the app's creators and see if that feature can be added to an update for it.