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I'm not sure of the procedure on a Windows device, but the first port of call would probably be a reset. Don't forget to back up your content first. If that doesn't solve it, then it'll need to go for repair, but it would get reset in that process anyway, so you've lost nothing.
The Early Termination Fee will be your montly payment minus the VAT and with a 2% discount. The exact sum will depend on when you do it, but you can get a pretty clear idea of it from that.
If you do go ahead, add a SIM-only deal to your account first and ask about porting your number to it if you want to keep it. If you cancel the contract, you'll lose the number straight away. Mind you, internal porting isn't always easy and you may have to go via PAYG first. 191 or Live Chat can quote you the ETF and should be able to progress it for you, but I'd be inclined to wait for the forum team to confirm the porting process - ie: don't do anything until you've spoken to them. They'll pick up your post in due course.
Bear in mind that you have a warranty on the device and you can ask Vodafone or Nokia to repair it. If you buy a device outright, it could still go wrong.
If a reset hasn’t fixed the issue, I’d recommend a repair.
If you want to upgrade early, as hrym says there’s an early termination fee.
Bear in mind that when you type a particular phone error into a search engine, it’ll come up with people experiencing the same issue. It doesn’t necessarily mean it’s a common fault, as it’s a small percentage of devices compared to how many are actually sold.
The Nokia Lumia 930 is a great phone. I’ve been really impressed when using one.
If your phone is faulty, there wouldn't be a cost for the repair unless it was damaged. In the event that Vodafone turn down a warranty repair (which I'm going to assume is unlikely), you can always go to Nokia who may take a more pragmatic view.