Carphone Warehouse, the official iPhone operator in the UK, has announced it is to offer cashback for old iPhones.

Offering possibly a quicker alternative to eBay, the retailer will shell out £100 for any capacity model of the first generation iPhone, £170 for the 8GB iPhone 3G and £180 for 16GB 3G.

To get the £170 and £180 for the 3G handsets, users will need to have bought out of their O2 contract - although presumably this will go some way to help cover that cost.

The details are still a little hazy, but we're told customers can sell in their old iPhones as a standalone transaction via Carphone's site, while in-store "it will need to be in conjunction with a cellular purchase".

We've done a quick check on online services that buy second hand mobiles, and it seems that for the 16GB iPhone 3G you're looking at anything from £200 to £250 (as a voucher option), while eBay could give you around £100 more on top of that, so it might be worth looking around before you commit to Carphone.