The only way AT&T will detect if you have the Galaxy S3 is if you put the AT&T SIM card in the phone, it will then go into their system.

If you do unlock the phone, AT&T cannot do anything against you for doing it, but I do believe your warranty would no longer be there if I am not mistaken

Unlocking the phone shouldn't void the warranty.  Only extensive unauthorized modification would void the warranty.

Not clear what you are trying to do.  If you run it on AT&T's network, then yes they can detect it from the IMEI number, which SIM unlocking does not change.

Thanks Redpoint!  I forgot about that part of the OP's question.  Yes, AT&T will be able to detect that it is a Samsung Galaxy S III.  I am using an AT&T branded GS III that is unlocked; my OLAM still shows that that line has a Samsung Galaxy S III on it, even though I have switched the phone off and back on to that line since I unlocked it.

I wouldn't assume they will unlock it.   I have been trying to get them to unlock my Skyrocket SII that I paid full price for.   They cannot.   I asked them if they could unlock an SIII, they didn't know.   They said they need an IMEI number to check if an unlock code is available.

That's true that paying full-price doesn't guarantee an unlock.  I paid full-price for my Nokia Lumia 900 and AT&T won't unlock it.

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Robert418 wrote:

I wouldn't assume they will unlock it.   I have been trying to get them to unlock my Skyrocket SII that I paid full price for.   They cannot.   I asked them if they could unlock an SIII, they didn't know.   They said they need an IMEI number to check if an unlock code is available.

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Its true that the IMEI number is needed to obtain the code.  The code is unique to the phone, and they have to enter the IMEI into their system to get the specific code for your phone.

 

The IMEI number of the phone can be found by going to Settings>About>Phone identity.

 

Its also sometimes printed on the phone itself.   Don't know about Samsung.  But HTC often has a label under the battery (take off the battery cover and remove battery).

You should just buy an international Samsung s3 it's quad-core and you won't have to worry about At&t unlocking it.