I can't install any Apps on my iPhone, over the phone network or WiFi, neither updates or new apps. After I type in my password (correctly) it goes to the home screen, briefly shows 'Loading', then I get the error 'Sign In Required. Tap Continue and sign in to view billing information'. I can't 'tap Continue' because the button is 'Ok', and it doesnt take me to any billing information, just closes the dialogue box. I've restarted and synced, but is there anything else I can try short of restoring it?