The mobile phone retailer said it will give people £170 for the 8GB version of the iPhone 3G and £180 if they trade in a 16GB capacity model. Anyone with a first generation iPhone will receive £100.
However, customers trading in an iPhone 3G must buy themselves out of their O2 contract.
The Carphone Warehouse said the handsets will be sold via its website, or in-store where they must be traded in against another mobile phone purchase.
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See also: In pictures: the Apple iPhone 3G S