Ultrasn0w 1.2.7 doesn’t come with any additional baseband support, but serves to unlock the modem firmware: 01.59.00, 04.26.08, 05.11.07, 05.13.01, 05.12.01 and 06.15.00.
If you are unsure as to what baseband your iPhone is currently running, you can check this by heading over to Settings > General > About and then viewing the Modem Firmware version number. If you are running iOS 5.1.1 with a compatible baseband (via Redsn0w/PwnageTool or Sn0wbreeze) then follow the steps below to unlock using the latest version of Ultrasn0w.
Step 1: Before users can take advantage of the software-based Ultrasn0w unlock, the device must be jailbroken.
Step 2: 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 7: Select the ‘Restart Springboard‘ option to reboot the device. Done!