According to info from DJI, nothing will happen when your phone rings until you answer it. They say the P3P will hover when you answer the call, but that is when the P3 is operating a function like POI. It does not say what it will do during free flight. Which is why I use a tablet and not just my phone. I am not willing to risk the unknown.
As for the GPS, In order to have that in a tablet it must have cellular capabilities. But I just tether my tablet to my phone and I still have GPS through the phone. My carrier is AT&T so I can use my phone and browse the Internet at the same time unlike other carriers. I'm not knocking the other carriers, I'm just providing information.