This could happen to you be careful:

SMS SCAMS

24 Jan 2006, 03:04:39 PM
Reporter: Helen Wellings
They're telemarketer's dreams - mobile phones generate fortunes for
scammers. With around 20 million mobile phone users around Australia,
those fraudsters can't lose. In fact complaints to the
Telecommunications Ombudsman about bills for premium rate SMS's have
tripled in the past year - and text spams top the list.
Gordon Renouf, of the Australian Consumers' Association, publishers of
Choice magazine, warns "There's more and more of this kind of spam
going to go out to consumers. It's only going to get worse unless the
regulations are fixed up."
The latest rackets attacking mobile users left, right and centre -
offers of so-called "free" ringtones, "you've won a competition for a
trip" scams and innocent-looking text messaging that tricks you into
replying.
"All the text messages were inviting and playful and much like a friend
would text you that's really what got me in," says IT expert Steve
Marr. He was intrigued by an SMS from someone say "Hi it's Susan. I'm
back. Contact me." Thinking it was an old acquaintance just back from
overseas, naturally he sent back a text message. "It came back with a
very bizarre message saying "blonde hair blue eyes long legs. Does that
help?" I thought I still don't know who this person is but they're
having a lend of me. Someone's got a sense of humour I'll play along."
After a few text messages, he discovered he couldn't make a call from
his prepaid mobile, even though he'd just bought $30 worth of credit.
Steve was scammed! By replying to the message, he'd automatically
become a subscriber to a chat service. Every time the service messages
him, it costs him $4 up; his replies cost him 50 cents.

Source: http://www.pbxinfo.com/forums/showthread.php?t=18067

So don't reply to any Text Message you get unless you know the person
for sure or you could be signing up for a chat service and not know it!