Overheating? wont make calls all-touch stops.

I had a customer bring in an iPhone 6 that gets very hot. I am wondering if its a lost cause, it turns on and stays on gets very hot the digitizer stops working, it wont make phone calls. its connected to wifi says service is there. i Tried a new battery and screen and the same thing still happens. i have checked their settings and nothing is set to stop calls. if anyone has any experience with this particular issue i would LOVE to know how you fixed it, or if its not fixable at all. THANK YOU