Few days back iPhone Dev Team has released the updated Ultrasn0w 1.2.1 unlock for Apple iOS 4.3.1 having old baseband and it also fix signal issues that were reported by iPhone users who had their iPhone 4, 3GS Jailbreak on iOS 4.3.1.

We’ve pushed out the 4.3.1 compatibility fix for ultrasn0w in Cydia — it’s now at version 1.2.1. If you’re not already at 4.3.1 and you need the unlock, please be sure you understand how to get to 4.3.1 using a custom IPSW that doesn’t update your baseband.

This isn’t a new unlock! It’s to allow those who are already unlocked to use FW 4.3.1. It also fixes the signal bar issue for those who aren’t using the unlock but retain an older baseband intentionally.

AFTER INSTALLING ULTRASN0W 1.2.1, PLEASE REBOOT YOUR iPHONE using the normal “slide to power off” swipe. T-Mobile users in the USA also should disable 3G mode in Settings->General->Network.

NOTE: Supported basebands: (01.59.00 / 04.26.08 / 05.11.07 / 05.12.01 / 05.13.04 / 06.15.00)

For unlocking iPhone 3GS, first you need to jailbreak it first and update its baseband to 6.15.00 baseband on iOS 4.1/4.2.1.

STEP ONE: You need to Jailbreak your Apple iPhone 4 / 3GS having iOS 4.3.1 by preserving your old baseband.

STEP TWO: Now you have jailbroken iPhone 4 / 3GS, unlock your iPhone 4 / 3GS using Ultrasn0w 1.2.1 by mean of Cydia. You can follow the steps below;

• Launch Cydia installer from SpringBoard.
• Tab on the Manage Button on the screen.
• Now select the source button.  
• Now press ‘Edit’ and after it ‘Add’. Same like before you will be prompted with a space to enter ‘URL’ of the source. Enter http://repo666.ultrasn0w.com and press on ‘Add’ to add it to repository.

Update will be done automatically by Cydia by following series of automated steps.

• After this all when installation is completed. Go in search and look for “Ultrasn0w 1.2.1” in Cydia and install it. This Cydia App will unlock your iPhone automatically, so that you can use it with any carrier.

STEP THREE: Here you are almost done, just simply reboot your iPhone 4 / 3GS and here you go. Now you have your iPhone 4 / 3GS unlocked on iOS 4.3.1.