Unfortunately Apple has made it increasingly difficult to unlock iPhones. Any iPhone 3G or iPhone3Gs(manufactured before the 35th week of 2011) can be unlocked no matter what by using 3rd party software called "Ultrasn0w." Ultrasn0w is only compatible with the following basebands:
- 01.59.00
- 04.26.08
- 05.11.07
- 05.12.01
- 05.13.04
- 06.15.00(iPad Baseband)
Chances are you won't run into anyone with 06.15.00 because if they had that baseband they would already know how to unlock(or someone they know would). If they have a 3Gs(again manufactured BEFORE the 35th week of 2011) and they do not have a compatible baseband, you can install the iPad baseband using Redsn0w and then use Redsn0w again to downgrade to 05.13.04(since the iPad baseband causes issues).
If they have a 3Gs that was manufactured on or AFTER the 35th week of 2011, unfortunately there is no way to software unlock, however, they can do an IMEI unlock which I will explain further down.
If they have an iPhone 4 that is on the 01.59.00 baseband, you can also use Ultrasn0w for that. However, that is the ONLY iPhone 4 baseband that is compatible. If they have a higher baseband version than 01.59.00 and lower than 04.11.08 you can use a SIM interposer. The most trusted and well-known of these interposers is the Gevey Ultra which is $23 from
Apple N Berry | Your iPhone Repair and iPhone Unlock Specialists
If they haven an iPhone 4 with 04.11.08 or higher, the only way to unlock is with an IMEI unlock(or if they are the original owner and the phone is from AT&T they can have AT&T unlock the phone if their contract is up).
For iPhone4S there are NO software unlocks available at all. The only way to unlock a 4S is with an interposer or IMEI unlock. As far as I know, the Gevey Ultra S($39.99 from AppleNBerry) can unlock any iPhone4S on any firmware/baseband combo up through 5.1.1 firmware. I'm not sure if iOS6 contains a baseband update for iPhone4S or if it would be compatible with Gevey or not.
For most phones that cannot be unlocked via software or hardware you can do an IMEI unlock which is basically where you pay someone with connections to add your IMEI to the whitelist which will permanently unlock the phone regardless of firmware updates. This can be quite spendy and usually can only be done on AT&T phones. I won't post any links to an IMEI unlock site because there are a lot of scams out there and I don't know of any reputable dealers.
Hope this answers some of your questions!
