Go to /private/var/root/Library/, delete original lockdown folder and put this lockdown folder after you extract it from the zip file
Restart phone
If you are not jailbroken:
Sherif Hashim discovered that you can activate your iPhone 4, iPhone 3GS or even iPhone 3G without the original carrier SIM card. You need a “phonebook sim card” or 02 simcard. You can buy these type of sim at radioshack, bestbuy or other mobile electronics store (for example here).
Once you get passed the Emergency Call screen, connect to Wifi, jailbreak using JailbreakMe.com and unlock using ultrasn0w. Once that is complete you should be passed activation and unlocked (able to use any sim). If you get a different simcard error, do the following final step:
Go to /private/var/root/Library/, delete original lockdown folder and put this lockdown folder after you extract it from the zip file
Restart phone
If you have itunes invalid response problem:
Connect to wifi and ssh to your phone (via winscp, putty, terminal, etc).
Navigate to your lockdown folder (/private/var/root/Library/).
Delete pair_records folder
Reconnect the cable and see that your invalid response problem is gone.
If you bought your iPhone 4 on ebay you will need the official sim (at&t) or a universal sim, like ones you can grab at bestbuy for $10 this will get you passed Emergency Screen.
Today Apple began selling iPhones without AT&T plan. Apple’s new policy is that anyone can walk into an Apple Store and pick up an iPhone 8GB for $499, 16GB for $599 and 32GB for $699. They allow one unit per customer per day, up to 10 iPhones.
The iPhones are still locked to AT&T’s network. Of course, these devices will be sold with the latest baseband which is not yet unlockable; however, you can use RedSn0w to perform a tethered jailbreak.