THis is a quote from Stevedalton from MSE, credit to him.
"Today I have just bought my wife a new PAYG phone from an O2 shop. The phone I purchased was a Motorola V8 flip phone. It was priced instore at £129.99 but on this phone, O2 stores are allowing £50 p/x on any old phone. I took up this offer and produced a very old Sony CMH333 phone - They used to be known as a Sony Mars Bar due to being the same size as a Mars Bar. It must have been nearly 15 years old and they currently go for around 99p on eBay.
Anyway, I handed over the phone and I had to sign a till reciept to confirm that I had recieved £50 trade in for it. I got my new phone for £79.99 (plus I had to buy a £10 top up but you can claim a refund on this provided you dont use any of the credit). When I left the shop I examined my reciept and clearly printed on that was a statement that by trading in your handset you are agreeing to transfer ownership of the handset to O2 for recycling and a few other conditions basically confirming that there is no personal data left on the phone, the sim has been removed, O2 can dispose of it etc. but the ones you will be interested in are the final 2 conditions and I'll type these exactly as they appear on the reciept;
4. I understand that in the case of any subsequent refund, this handset cannot be returned to me and a refund will be given to include any allowance obtained by transferring ownership of this handset to O2 UK.
5. I UNDERSTAND THAT TRADE-IN HANDSETS CANNOT BE RETRIEVED OR RETURNED TO ME UNDER ANY CIRCUMSTANCES.
All you have to do now is to return your recently purchased Motorola handset under the 14 day returns policy, claim a refund on what you actually paid for the phone and according to their own terms, also claim back the £50 trade in allowance for your p/x phone as they cannot return it. Easy peasey."