Try to reset the cache on your voice call app. If that dosen’t work, reset the data of your voice call app (this will delete all your call history/contacts). If still no success, reinstall your voice calling app.
Repair it yourself. That’s the point of this website.: You haven’t provided enough information to know what’s wrong with it, but I recommend you just repair the thing yourself. It’s the point of IFIXIT.
It’s hard to know for sure what’s wrong with it, but if you have some extra parts swap them out one by one and see if you can find out what’s wrong. You also might have a bad screen; your device could be booting, but it just dosen’t show it on the screen.