When I try to use my iPhone 4 on the internet w/ 3G not WiFi I regularly (but not 100% of the time) get the error message "Could not activate cellular data network: You are not subscribed to a cellular data service". The problem is that I AM subrscribed to a cellular data service (Nationwide Family 700 with Unlimited Data/Text/Pictures) and my wife's iPhone 4 works fine. Any suggestions?