I have seen that there are many other model numbers of iPhone 4S's, but I have not seen any info about model number MC922ll/A
Is an iPhone 4S of this model unlocked or locked to a specific mobile carrier?
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If it's anything like it is here in the UK, it's the device's UID that determines whether it is locked - Apple have a massive database of the UID's along with references as to whether they are locked (this is why warrantee swap-outs always come out with the same lock).
Normally contacting the networks, if you're a customer in 'good standing' they'll let you unlock for 'international use' - which is exactly the same as unlocking the phone.
AT&T has a site to check and ask for device unlock:
It appears from various sales websites that this model number is locked to AT&T. (Source)