I recently got a new Vodafone contract. They filter their internet content for adult material such as gambling websites and porn. The way to do this is on your online account. However, they have to take £1 off a credit card to prove you are 18. To compensate you for this, they put £2.50 credit on your account, giving you a net gain of £1.50. Ok, it's not much but it all counts!
Note: I think you can do this by calling up too.