My iPhone says it needs to be restored, but on connecting to iTunes i get unknown error, and unable to restore: any tips?

On 2 occasions my iphone has come up with the message that it needs to be restored to factory settings, but on trying to do this, itunes displays the message that it is unable to restore phone due to unknown error.  Any tips?
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Unless you have a jailbroken phone that has altered the baseband, that usually a hardware failure, in which case you'll have to take it to the Apple store for evaluation.