I'm having the exact same problem... minus the freezing when I make a call. But once on a call for a few mins and go to hang up, the screen is frozen. I can tirn it off and back on, but its still frozen. I have to hold down power button and let it reboot to fix it. It also happens ever so often if accidentally hold the back button and it opens multitasking. The screen will freeze. When that happens though, I can hit the home button and all is fine.
A little back story, I had just replaced the screen after it broke. Powered up phone,then let it update to denim. I never had this problem before denim (was on Cyan still when screen broke), so I'm starting to think its denim. I've already reflashed with Lumina tool and hard reset, restored backup and still the same problem. I'm now trying to remove apps one by one to see if any of them are causing it possibly. Finally, I may just reflash or hard reset and not restore a backup to see if its just denim and not an app.