How to Unlock iPhone 3GS on iOS 4.3 With Baseband 05.16.01

To unlock an iPhone 3GS with baseband 05.16.01, you cannot use Ultrasn0w directly because that baseband version is not supported for software unlocking. The standard procedure involves using a tool like Redsn0w to update your baseband to the 06.15.00 iPad baseband and then immediately downgrading it to version 05.13.04, which is compatible with the Ultrasn0w unlock. Please note that this process carries risks, particularly for iPhone 3GS models manufactured after late 2011 (check if the 3rd, 4th, and 5th digits of your serial number are higher than 128), as the iPad baseband can brick those newer radios. A safer and more permanent alternative is to request an official IMEI unlock from your carrier. If you decide to update your firmware, the final supported version for the iPhone 3GS is iOS 6.1.6, which can also be jailbroken and unlocked using the same baseband downgrade method.