It's a 3G, I use Windows, and modem firmware 05.12.01
There's no unlock for firmware 3.1.3 on baseband 05.12.01.
You must downgrade the baseband to 04.26.08 first.
Would you like to try?
*Downgrading will only works with (bootloader)05.08. if you have 06.4 (bootloader) it's not going to work.
How to Downgrade iPhone Baseband from 05.12.01 to 04.26.08
Step 1: In order to downgrade your iPhone baseband from 05.12.01 to 04.26.08, you must first jailbreak your iPhone using RedSn0w or Sn0wBreeze.
Step 2: After jailbreaking, launch Cydia, then search for and install Fuzzyband.
Step 3: Launch Fuzzyband then tap “Downgrade”, which will start the downgrading process.
Step 4: Reboot your iPhone.
You should now be able to unlock using UltraSn0w. If you’re not sure how to do that, check out this tutorial on how to unlock iPhone 3.1.3.
How to unlock iPhone 3.1.3 with UltraSn0w
* Launch Cydia.
* Add the following source to Cydia repo666.ultrasn0w.com (note there is a “0″ in sn0w, not an “o”).
* After installing this source in Cydia, search for “ultraSn0w”.
* Install UltraSn0w and reboot your iPhone.
*Sn0wbreeze.
http://ih8sn0w.com/index.php/welcome.snow *Redsn0w
http://iphwn.org/redsn0w-win_0.9.2.zip