This is how I got BT Fon to work on my iPhone4.
1. Download BT Fon app
2. Log in using your bt internet email address and password
3. Click on 'Opted in' and Agree to the terms and conditions - you should only have to do this once.
This should get you up and running with BT Fon. It's the Opt in part that BT does not make obvious.
Hope that works.
hi i had the same problem with my iphone 4 i had to change the security settings from wpa/wpa2 psk to wep and set my own password that way it would connect and still have the security hope this helps.