How to Unlock iPhone 3GS on T-Mobile Using Unlock Code?

Hello Mary Ann, unfortunately, there’s no way you will be able to unlock your old iPhone 3GS (even its out of contract) with just a code. Because of the fact that there is no such thing as unlock code. According to AT&T’s FAQ page, iPhone cannot be unlocked, even if you are out of contract. But if you really want to unlock, then you will be glad to know that there is an alternative. But, of course, you will need to jailbreak your iPhone 3GS in order to do it.

Since, according to you, your iPhone 3GS is untouched or never been jailbroken, then there’s a possibility that you will be able to jailbreak and unlock it. But just to make sure, you must check the baseband of your iPhone 3GS if it’s unlockable or not. To do that, just simply tap on “Settings” and then tap on “General” and onto “About”. Then slide down and look next to “Modem Firmware” – these numbers will be your baseband version. iPhones with baseband 06.15.00 / 01.59.00 / 05.13.04 / 05.12.01 / 05.11.07 / 04.26.08 can be unlocked using Ultrasn0w.  If you have baseband 05.16.02 / 05.16.01 / 05.15.04 / 05.14.02 on your iPhone 3GS, then you will need to upgrade it to iPad ‘s 05.16.02 baseband using Redsn0w 0.9.6rc16 in order to unlock.

You can follow this tutorial on how to update the baseband version of your iPhone 3GS and jailbreak it on iOS 4.3.3 using Redsn0w 0.9.6rc16. Then once jailbroken, you can follow this tutorial on how to unlock iPhone 3GS using Ultrasn0w 1.2.3.