Aw we all know , the iPhone Dev-Team has released the Ultrasn0w unlock for iPhone users. Recently they updated the UltraSn0w iPhone unlock for iOS 4.2.1.The unlock will work on iPhone 4 baseband 01.59.00 and iPhone 3G/3GS basebands 04.26.08, 05.11.07, 05.12.01, 05.13.04 and now 06.15.00 and this thing confusing users because iPhone Dev Team has added a twist of upgrading your iPhone baseband to 06.15.00 for those users who has not above mentioned baseband.
So if your iPhone 4 is not on baseband 01.59.00, you will not be able to unlock it.If your iPhone 3G or 3GS is not on one of the baseband version mentioned above, you will have to first update your baseband to 06.15.00 See our complete guide on it.
How To Unlock Your iPhone 4.2.1 With UltraSn0w:
Step 2: Launch Cydia, and search for UltraSn0w.
Step 3: Download UltraSn0w.
Step 4: Reboot your iPhone
Done, Now your iPhone is unlocked, test it by poping some other carrier sim on it!