I just bought a used iPhone 3gs that I'd like to restore to the factory settings. Is it safe to erase & update using iTunes if I intend on unlocking it to use on T-Mobile? It currently uses 4.1 & 06.15.00. Any and all advice is appreciated. ETA- Also, the seller negelected to tell me his passcode to reset using the Settings menu.. so until he gets back to me (if/when), I'm kind of stuck :/ ..unless I can bypass that somehow?