If you want to sell your phone, just get it unlocked (£5) and stick it on
Ebay. You'll get far more.
We once had new phones from O2, two brand new top of the range ones, this was about 3 years ago.
The order was messed up and they actually sent us 4 of one phone and 3 of the other.
I called O2 straight away and explained they had sent too many and advised that if they would like to come and pick them up I'll make sure they're available (but insisted I would not take them to the post office as i'm not putting myself out for their mistake).
They never turned up to take them back nor did they contact me about them, and after another phone call and a wait of 6 weeks, I had given them more than enough time and they obviously didn't want them back
all the phones were sold on Ebay.
Had a nice week away in the canary islands on that.
Cheers O2!