Just tried to follow one of Charlotte's links (to a wallet, nothing dodgy) with my iPhone, and got redirected:
The site you tried to visit is restricted to over 18s.
To remove the customer protection block all you need to do is to confirm your age. You can do this by either calling our free automated service 61018 or continue to our online service.
...and then it goes on about needing my MasterCard details!
Is this a scam, or is it genuine? Are O2 going to ask me to "verify" myself every time I look at various sites (it's a wallet, FFS!)? Are they going to charge me for this priviledge? Which other PAYG providers would people recommend instead for a smartphone-user who only uses texts and web?