It's the same whenever you text an organisation such as the BBC or Five (I'm thinking of the competitions they run after shows like CSI) and why they state the mobile phone charges will be higher.
I fell for it once with the BBC's 'Test the Nation' thing they ran a few years ago. I texted my answers, and got a thank you text for submitting them - only to find out that while I had knowingly paid 10p per text to respond, I got charged 12p for the response from the Beeb thanking me for taking part and acknowledging they had received my answers. I've never done it since.
In a similar thing to what Death Axe was referring to, a friend's young daughter (11 or 12 at the time and not particularly streetwise) got sucked into one of the music channels and the way they did it was to reply to her as though they were her friend, being really interested in her and asking her to tell them more about herself, etc. It was only when her brother spotted it scrolling across his TV in his own room, and recognised certain things, that they realised and the bill came in at over £300. Personally I think these sorts of things should be banned, along with Finance/Loan adverts, betting adverts, 'get the value of your car'-type adverts ... this list is endless
Sheffield is the centre of all greatness in the Universe (c) Jarvis Cocker 2006