Only works for modem firmware: 01.59.00, 04.26.08, 05.11.07, 05.13.01, 05.12.01 and 06.15.00. To find modem firmware version go to settings-->general-->about-->modem firmware.
Step 1: Before users can take advantage of the software-based
Ultrasn0w unlock, the device must be jailbroken. The first step in the
process is to jailbreak iOS 5.1.1 device with older baseband preserved via a custom firmware built using either the latest version of Redsn0w/PwnageTool or Sn0wbreeze.
Step 2: When the jailbreak process has been completed, the Cydia icon
will be present on the home screen. Launch Cydia and wait for all data
to reload from the default repositories.
Step 3: Cydia provides content to users that are hosted in specific
repositories. Due to the fact that Ultrasn0w is provided by the iPhone
Dev Team, it is hosted on one of the default repos that come
pre-installed with Cydia, meaning no additional repository needs to be
added. Select the ‘Search’ option from bottom navigation bar and type in
‘Ultrasn0w’ before hitting search.
Step 4: For the sake of education, if for some reason you actually
need to enter the repository it can be achieved by selecting the ‘Manage‘ option followed by the ‘Sources‘ button and then selecting ‘Add’ in the top left hand corner. In the pop up box which you will see on screen, enter ‘http://www.repo666.ultrasn0w.com‘ and hit ‘Add Source’.
Step 5: Select the Ultrasn0w package from the search results list and select ‘Install‘ on the top right hand corner of the interface.
Step 6: Confirm the installation of the package and allow the installer to go to work until it prompts that it has finished.
Step 7: Select the ‘Restart Springboard‘ option to reboot the device. When completed, the iPhone should be unlocked for use with any network of your choice.