What is the Best Option to Unlock iPhone 3GS on iOS 4.3.1?

Hello Anthony, well, the very first thing you need to do is to jailbreak your iPhone 3GS on iOS 4.3.1. Unfortunately, there’s no way for you to upgrade your iPhone 3GS to iOS 4.3.3 due to the fact that the Cupertino, California-based Apple already stopped signing iOS 4.3.3 firmware to prevent iOS device users from jailbreaking their devices using JailbreakMe 3.0. Anyway, in your case, the only option available to unlock your iPhone 3GS is by updating the baseband of your device to iPad’s 6.15.00 baseband. However, you should be note that updating the baseband of iPhone 3GS to iPad’s baseband will automatically lose the GPS functionality of your device. This also means that all apps that utilizing GPS will not work as well, and there’s no fix for this bug at the moment.

Anyway, you can use Redsn0w 0.9.6rc19 (download it from here if you are on Mac OS X or using this link if on Windows) to jailbreak your iPhone 3GS on iOS 4.3.1 and update its baseband to 6.15.00. You can use our step by step guide posted here on how to jailbreak iPhone 3GS as well as upgrade its baseband to 6.15.00 using Redsn0w. Then once completely jailbroken and running on 6.15.00 baseband, you can now unlock it using the latest version of Ultrasn0w carrier unlocking tool. You can use this tutorial to unlock iPhone 3GS using Ultrasn0w 1.2.3.