If you're wanting to exchange your old mobile phone for cash or shopping vouchers, I would definately recommend this website. They sent me a freepost package to post my phone to them and I received payment within a few days. They also kept me updated with texts- letting me know when they'd received my phone and issued payment. Thumbs up!
If you're looking to exchange your old mobile phone for cash DO NOT USE 'SELL OLD PHONE' (www.selloldphone.co.uk). This website is a complete SCAM. I was offered £40 cash cheque in exchange for my old phone, which was a fairly new nokia. I posted the phone to the freepost address given and never received any payment. No-one answers the customer help line and e-mail responses just claim that payment has already been issued. It's impossible to speak to anyone. Just don't do it!