How do unlock codes work?

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@rochrunner wrote:

My Samsung S2 Skyrocket is eligible for unlocking, although I actually haven't used it for a while. But I want it unlocked so I could use it, for example, while traveling to Europe where I can get a prepaid SIM card.

 

I applied for and received my unlock code from AT&T, which requires me to insert a non-AT&T SIM card. Trouble is, I don't have such a card at this time! I can probably go to a local cell phone store and ask to borrow one for the unlock process, but I am also wondering about this somewhat ambiguous sentence in the email that I received:

 

"Please allow 24 hours upon receipt of this notification to complete the unlock process."

 

I'm not sure if ths means the code is only good for 24 hours (hope not!!) or that I should wait 24 hour before trying it.

 

My problem is that, thinking about this from a technical standpoint, how does this code work? If I insert a non-AT&T SIM card and enter the code, how does the phone "know" what the correct code is, assuming that this is a custom code that will only work on my phone? As I said, there is no SIM card in the phone at all right now and it's not even powered on, so there's no way for AT&T to send anything to it in advance.

 

Can someone tell me how this works before I waste a trip to a local dealer?

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For phones that require an unlock code, such as yours you can unlock as soon as you receive the code. Swing by a T-Mobile store, pick up a SIM card and enter the code when prompted. The code is not going to expire in 24 hours or ever.

The 24 hour window is probably iPhone related as the unlock is performed by the iTunes, there's no unlock code to enter.