Sorry to hear you've had some issues with the phone. I believe this to be a device issue. We may need to bring this in for repair. You can book this in by logging in to your online account here and accessing the repair option in Plans and Devices > Device Repair.
You can also call us on 789 or 0345 6000 789 and we'll look into this for you as soon as possible. Let me know how you get on.