The unlock tool ultrasn0w has just been updated to version 1.2.8 thanks to iPhone dev team member Sherif Hashim. However, this update is compatible with iOS 6, 6.0.1, 6.0.2, 6.1 on older basebands only and does not unlock the new baseband versions. If you don't know what ultrasn0w really is, it is a free unlock tool so you can use your iPhone with any carrier of your choice. Read more for the complete guide with images.
UltraSn0w 1.2.8 supported basebands/modems 

  • iPhone 4 : 01.59.00
  • iPhone 3G/3GS : 04.26.08, 05.11.07, 05.12.01, 05.13.04, and 06.15.00

Note : To determine you can unlock your idevice, you need to check your baseband version go to Settings.app > General > About > Modem Firmware 

 How to Unlock iPhone 4,3GS On iOS 6.x Using ultrasn0w 1.2.8

Step 1: Your iPhone 4, 3GS and you must also be jailbroken on iOS 6, 6.0.1, 6.0.2, 6.1. See the full step by step tutorial for evasi0n jailbreak tool for Mac and for Windows.

 Step 2: Launch Cydia from your springboard. Tap Manage at the bottom of the screen > Sources. 

Step 3: Tap on Edit > Add. Now add this url : http://repo666.ultrasn0w.com/. Now tap on Add source and tap Return to Cydia. Wait as Cydia refreshes and updates the sources.

Step 4: Tap Search tab at the bottom and then search for ultrasn0w. As you find it, tap the install button. This is going to install ultrasn0w 1.2.8 software tool to your iPhone on iOS 6.1. 

Step 5: Reboot your idevice, insert the SIM card of your desired network and wait for it to catch the signals. 

 Congratulations ! Your iPhone has just been unlocked successfully on the iOS 6.1 firmware version.

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