If your phone meets the following requirements it can be unlocked:
1. Jailbroken
2. OS 2.2
3. Baseband 2.28 or earlier
If you have met these requirements you need to open cydia and install the program “yellowsn0w”. You can download it by adding the yellowsn0w repo to your Cydia sources.
The repo URL is http://apt9.yellowsn0w.com.
once you have downloaded yellowsn0w and installed you are ready to use your iphone on any GSM carrier.
If you have 2.2.1 your options are as follow:
If your phone has the latest firmware 2.2.1 and was jailbroken using quickPWN and has boot loader 5.09 your pretty much SOL.
if your phone was jailbroken with quickPWN but has bootloader 5.08 you may be in luck.
if your phone was jailbroken with PwnageTool and had the firmware customized and retained the baseband just install and run yellowsn0w.
if your phone was jailbroken with PwnageTool and didnt retain the baseband but has bootloader 5.08 you may be in luck.
If you are using 2.2.1 with the latest baseband and have have bootloader 5.08 your are in luck, you can downgrade the baseband using a simple tool “3G Fuzzyband Downgrader” that is available through cydia. If you have bootloader 5.09 you can download this and attempt to downgrade your baseband but you will just get a you fail message [actually tells you if your capable of downgrading when you open it now]. Once you have downgraded your baseband just install “yellowsn0w” and your unlocked using 2.2.1
If you are unfortunate enough to have 5.09 and 2.2.1 you are still able to unlock your phone using a periphial device such as a turbo sim, rebel sim or equivalent.