If you did something right you might want to share your success with others. That is what I want to do here. I know how to unlock iPhone 3G 4.2.1 baseband 05.15.04 and I'll show here how to perform it. Some my old friend asked me to unlock his iPhone and I agreed. The first thing I did  I checked the baseband and it was 05.15.04 and the firmware was iOS 4.2.1. It has become clear to me what to do next. There are three actions that you must to perform to unlock iPhone 3G 4.2.1 baseband 05.15.04:

1. Download iOS 4.2.1.firmware.
2. Update 05.15.04 to 06.15.00 (iPad baseband) using latest Redsn0w.
3. Downgrade iPad baseband to 05.13.04 using the same Redsn0w.
4. Unlock 05.13.04 using latest Ultrasn0w.

Let's do it one by one.

How to Unlock iPhone 3G 4.2.1 baseband 05.15.04

Note: Before we start you must be sure that your iPhone is jailbroken. Step 1. Download iOS 4.2.1 ipsw file here. Step 2. Download latest Redsn0w which is now 0.9.14b2. Step 3. Extract and Open Redsnow. Note: If you are Windows user open it as Administrator. Step 4. Go to Extras => Select IPSW and choose your downloaded file in the first step. Step 5. After Redsn0w detects your firmware click Back and then jailbreak. Wait till it will ask you to Turn off iPhone and connect it to the Mac or PC. So do this using Slide to power off option. Step 6. Now you need to enter DFU, so if you fill strong in this just click Next and follow on screen instructions. But if you need help in this you can use this link that will help you. Also if you have your Home or/and Power buttons broken you can go here to enter DFU mode automatically. Step 7. Now you will see a list of options. All you need to do is to uncheck all off then and select only Instal iPad baseband which is need to unlock iPhone 3G 4.2.1 baseband 05.15.04. Step 8. Now just wait and watch how Redsn0w doing his work. Step 9. Now wait when your iPhone will reboot and check your baseband Settings => General => About. If it is 06.15.00 you can be sure that you have just successfully update your baseband. Now the most interesting. You can choose to retain iPad baseband because now you can use Ultrasn0w to unlock iPhone 3G 4.2.1 baseband 05.15.04. But there is better way to downgrade iPad baseband to 05.13.04 which is 100% unlockable. To perform it follow steps below which is the same as in the first part of this tutorial: Step 1. Open Redsn0w and go to Extras => Select IPSW and choose iOS 4.2.1 file which is saved on your computer. Step 2. Click Jailbreak, turn your iPhone off and connect to Mac or PC. Step 3. Enter DFU mode. Step 4. If you see list of options select only Downgrade iPad baseband and click Next. Step 5. Wait till Redsn0w finishes the process and reboot your iPhone. Now just check which baseband you have now. It is great if it is now 05.13.04! If not then just use this link that contains more information about 06.15.00 downgrade. As for me, first I got No WiFi, No Bluetooth, No baseband so I started from the beginning again. First I updated iPhone 3G to iPad baseband then downgraded it to that baseband which I needed. And it was successfully. Now it is the last thing to do.

Unlock iPhone 3G iOS 4.2.1 On Baseband 05.15.04

Step 1. Remove your Sim card, open Cydia and go to Search. Step 2. Find Ultrasn0w which is 1.2.7 for the time writing. Step 3. Install it and click Restart Springboard. Step 4. Now put your Sim card in iPhone and wait till it connects to your network. Note: If you will get No Service go here to fix it. Also you can you can use full tutorial guide how to use Ultrasn0w. And that's it. Now you can use your unlock iPhone 3G 4.2.1 baseband 05.15.04 without any problems. I hope to do this will be easy for you as it was for me. Tell me about it using comment below.