Each time you make a top up on your O2 pay as you go phone, you will be sent a code to input into the o2 top up surprises web site. When you input the code into the website you will be given either free mins, texts, mms, wallpaper or even free game. so if you are going to top up £20 it might be an idea to do two £10 top ups. This website isnt live at the moment, but, is meant to become live on monday.
I have had training on this promotion and its meant to be live on 24/11 for 1 month.