從我本人的英文博客轉載過來的，適合於使用美國AT&T 2年合約iPhone，並且合約期滿官方解鎖的朋友參考。特別是一般在AT&T客服聯繫蘋果公司官方解鎖後會要求你把iPhone鏈接iTunes restore，然後iTunes會要求你升級到你升級到最新操作系統，而這可能並不是你要想到的。本文最後提供的幾個步驟可以幫你繞開“升級”這一步。
I got my iPhone 4 with AT&T's 2-year contract in 2010, and recently upgraded my line to iPhone 5. This way the iPhone 4 is off-contract and qualifies AT&T's unlock eligibility.
Unlocking an iPhone allows you to use SIM cards from local wireless carrier when travelling aboard, which are usually cheaper than paying for international coverage from AT&T. You can also use the phone on other GSM carrier (CDMA in Verizon's case) in the United States, such as T-Mobile, although data speeds might be slower. If you want to sell used iPhone on eBay or craiglist.com, unlocked phones are usually more popular.
After I decided to unlock my iPhone 4, I looked up online and here's my procedures.
1. Look up iPhone's IMEI number
Go to Settings, then General, then About, write down the IMEI number.
2. Call AT&T customer service
Customer representative will verify your information and check unlock eligibility. In my case, it only took less than 1 minute before the representative told me she had unlocked my iPhone through Apple.
At this moment, she told me to plug iPhone into computer and connect to iTunes, do a backup, then click restore. The restore process will trigger the unlock.
So I plugged my phone, did a backup, and hesitated when trying to restore, because it says my iPhone 4 will be restored to latest iOS version which is iOS 6. Considering there's no siri with iPhone 4, there really isn't much features to get from iOS 6. Then I wonder if there's a way to unlock iPhone without restoring and looked up online again. Luckily got to this post.
I would like to endorse those steps here and add a couple of screenshots that were captured in my unlock process.
1. Download latest version of redsn0w software at here
2. Run redsn0w on your computer
3. Connect iPhone to computer. Step 4 to 8 shows how to put iPhone on DFU mode. iPhone is connected to computer during the whole process.
4. Turn off the device.
5. Hold the power button for 3 seconds.
6. Hold the home button without releasing power button for 10 seconds.
7. Release the power button but keep holding the home button.
8. Keep holding the home button until you are alerted by iTunes saying that it has detected a device in Recovery mode. Make sure the device screen is blank and no logos (including Apple logo) are present.
9. When redsn0w recognized your iPhone in DFU mode. Choose Jailbreak.
10. Options will appear with checkboxes. Make sure the only box checked is Deactivate then click next.
11. Keep iPhone connected with computer. iPhone will flash words on the screen then reboot.
12. After the iPhone rebooted, open iTunes on computer.
At this moment, you should receive a message in iTunes confirming that your iPhone is officially unlocked.
