The offer, exclusively available from Carphone Warehouse, is available on either the white or black phone and is subject to a mandatory £10 top up at checkout. You can also get £3.03 cashback on your purchase from TopCashback.
We've talked a fair bit about the Lumia 620 before - it's a great little handset even without this offer! The previous deal from O2 offering a free additional coloured shell rather than the Windows upgrade is still available (as detailed in our previous post) if that appeals to you more.
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