This guide is for those who have iPhone 3G and 3GS [ older bootroms ] with firmware 3.1.3 who have jailbroken their iphones with Sn0wbreeze which does not upgrade your baseband so the baseband should be 04.26.08 and make sure you check the baseband version before you proceed with the tutorial below.
Unlock iPhone 3G, 3GS With 3.1.3 at 04.26.08 Baseband
1. Firstly Download Sn0wbreeze 3.1.3 and jailbreak your iPhone 3G & 3GS 3.1.3 Firmware
2. After Jailbreaking run Cydia on your iPhone, tap manage tab
3. Tap edit and add http://repo666.ultrasn0w.com source url and click on Add sources.
4. Now search for ultrasn0w 0.92 and install it.
5. Now restart your iPhone and after restart you can use sim card of any cellular carrier in your iphone, which means it has now been unlocked.
[ Source Redmond Pie ]