I have just realised I had this problem but a few searches on Three's site and I found a page that states that they are improving access to all by removing this restriction filter. The removal states it is from March for a selection of phones and from July for all phones.
I'm new to this forum so will attempt to paste Three's statement here. If this doesn't work then go to the Three support site and search on "Age restricted websites on your iphone".
I rang them today and got the bar lifted on my Sony X8.
Accessing age restricted websites on your iPhone.
Why can’t I access certain websites, such as gambling or adult sites?
Our network uses filters to block access to age restricted websites, including adult content, gambling, some personal blogs and file-sharing sites. We’re now updating the way we handle internet traffic to improve the experience for all our customers so you’ll get open internet access without filters.
From March 2011, you will have open internet access on these smartphones. You also need be over 18 and not have an active filter set up on your account. If your phone is not on the list, please bear with us while we update our data settings. You’ll have open internet access on any phone from July 2011.
Note that you need to be over 18 to have the filters removed. If you’re on Pay As You Go and you haven’t previously been age-verified to use the Top Shelf service, you’ll need to have a credit card in your name to remove the filter.
Why do I sometimes get blocked, but at other times I can view the same site?
We’re updating the way we route internet traffic on our network. While we’re doing this work, you may occasionally get routed through a path that blocks access to certain websites. If this happens, switch off your phone and then switch it on again, or switch it into flight-safe mode and then back – this will reset the connection. This is a temporary problem and shouldn’t happen after 11 April 2011.
How can I apply a filter to block access to age restricted websites?
All you need to do is text FILTER to 33333. The filter will be applied within 72 hours.
Note: if you’re using a BlackBerry, we can’t apply a filter on your phone. This is because BlackBerries use different settings to access the internet.
How do I remove the filter?
As long as you’re over 18 and using a phone that’s listed below, just call us on 333 and we’ll do this for you.
If you’re on Pay As You Go and haven’t previously been age-verified to use the Top Shelf service, you’ll need to have a credit card in your name to remove the filter.
Open internet access will at first become available on these smartphones. If you don’t have one of the phones on the list, please bear with us while we update our data settings. You’ll have open internet access on any phone from July 2011.