Disable iPhone 3G GPRS or Edge Connections
This is a great trick if you have kids that play with your iPhone and you’re worried about them using the network. Tap into your network settings (Setting > General > Network > Cellular Data Network). In the “APN, Username and Password” field, type in any words so that your iPhone will not pass the correct values to the service providers thus preventing network access. You may have to restart your iPhone after changing the values.
