i bought a used iphone 3g which has a firmware of 3.1.3, i am wondering if anyone could give me a good software that could unlock my phone? or is there a better software if i update the phone to higher version to unlock?
Disregard the cannot unlock part, that is old. As long as the phone has never been on 4.1 then you can unlock with ultrasn0w.
Also if the phone is already activated you can alternatively jailbreak by going to jailbreakme.com in safari on the phone. It is a much easier jailbreak.
So you jailbroke and have cydia on the phone now right? Go to settings-general-about and scroll down til you see modem firmware and post what it says there.
There is your issue. That modem firmware cannot be unlocked right now. At some point that phone was updated to 4.1. You will have to wait for the new unlock to come out which could be as early as this week.