Unlock iPhone 3GS Softbank iOS 6.1.2?

[mnya]

Would you please help me figure out if & how unlocking my iPhone is possible?
- 3GS
- Softbank-locked (I still have the SIM)
- iOS 6.1.2 (I did not save iOS 4 / 5 SHSH blobs when updating)
- 16GB

I want to unlock it so I can use it on a prepaid plan with a carrier in the U.S.
Ideally I would like to have all the functions like data, call and text.
Thanks!!!

 

[luigismartphone]

Jailbreak it with Redsn0w and flash the 06.15.00 iPad baseband (also with Redsn0w) and install Ultrasn0w from Cydia or buy an IMEI unlock from http://www.officialiphoneunlock.co.uk