Fuzzyband has been updated to include downgrading support for iPhone OS 3.1.3, to downgrade iPhone baseband 05.12.01 to older 04.26.08 baseband that can be easily unlocked but, only for 05.08 bootloader users (Early iPhone 3G’s).
You can find Fuzzyband in the system section of Cydia, this utility helps early iPhone 3G users to downgrade their baseband to an unlockable old version. Allowing them to run OS 3.1.3 even if they accidentally upgraded but, it won’t work for iPhone 3GS.
If your iPhone meets the criteria you can use latest version of Fuzzyband which supports iPhone 3.1.3 to downgrade your 05.12.01 baseband to an older 04.06.08. Once you have downgraded your baseband you can use ultrasn0w to unlock your iPhone on iPhone 3.1.3 firmware.
How to Downgrade 05.12.01 Baseband to 04.26.08 Baseband
In order to downgrade your baseband 05.12.01 to older 04.26.08, you must first jailbreak your iPhone 3G running firmware 3.1.3 by using these step by step guides: redsn0w 0.9.3, or redsn0w 0.9.4, or snowbreeze v1.4, or Pwnage tool 3.1.5.
After you have jailbroken your iPhone, go to “Cydia” on your iPhone and Search for “Fuzzyband” then install the application.
Note: For those of you who do not know their bootloader version, simply run Fuzzyband and it will report the version number.
Start “Fuzzyband” application and touch on “Downgrade” button to downgrade your baseband 05.12.01 to older 04.26.08.
Your iPhone has been downgraded and now you can unlock your iPhone by using ultrasn0w to work with any carrier SIM.
You can follow our guides on jailbreaking iPhone 3.1.3 firmware for iPhone 2G, 3G, 3GS and iPod 1G, 2G using redsn0w 0.9.3, redsnow 0.9.4, snowbreeze, Pwnage Tool 3.1.5 and can unlock your iPhone by following ultrasn0w and blacksnow guides.