I have a new iPhone 4 purchased from Apple/ATT. I want to use a sim card in China for calling local phones and wifi, etc. Is it possible to unlock my phone and use a Chinese mobile sim card while in China? If I do this can I still update with system updates or must I re-lock the phone to update the system when new patches are provided? Is it possible to restore the phone to the original locked configuration? Will I void all warranties if I do this?