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Specific, standalone phone insurance will be more expensive than an extension to your home contents insurance, but more comprehensive: ie you'll get cover for a wider range of fuck-ups and less likely to get bitten by the small print, but it'll cost you more.
Check who the underwriter of the policy is: "cheap phone insurance 4 u" will just be an agent for the actual insurer and don't write the cheques.
I have it through my bank account and they seem to have contracted it to Aviva. I have used it twice and both times Aviva have been utterly outstanding.