help needed

Hi all

No 1 son (15 y.o ) was looking at a few adult web sites on the internet a few weeks ago and yesterday I received a bill from Telstra for just under $100. Apparently some sites he visited were the kind you pay per minute. After initially denying he was responsible (he tried to blame his 9 y.o brother) he confessed. None of the sites he viewed, he assured me, stated anything about costs being involved.

Is this legal? Can someone be charging people to view their website without informing them? For what its worth, I am not with Telstra, but the site he was viewing dialed an overide on my phone line and thus the bill came from telstra. I called telstra and the guy I spoke with said that they have been charged this money, and they are now charging me, and more or less said that I have to pay.
I guess I am hoping there is a way I can get out of paying $100 for my sons foolishness. I think it stinks that someone can do this to me or anyone else and get away with it.

Thankfully I have 1900 numbers blocked, as this could have been very very expensive ( I was charged o/s rates to the UK )

Any assistance would be appreciated

Thanking you all in advance

If you do have teenagers be aware that they can get up to mischief on the internet, so please consider blocking both 1900 and o/s access.