Your iPhone 4S Is Never Truly Unlocked
From How U.S. Carriers Fool You Into Thinking Your iPhone 4S Is Unlocked, or from the “There oughta be a law” department:
Last month I purchased an iPhone 4S from Best Buy Mobile for use on my Verizon account. I paid full retail so there would be no restrictions from Verizon with regard to unlocking the handset. My intent was to run tests in Europe and the U.S. to determine if the phone would perform as well as competitive CDMA-GSM phones that I have used (Android and Blackberry). Initially, my focus was on how well Apple handled the different issues required for roaming on foreign carriers and whether the Apple iOS 5 offered the same level of connectivity and data options that Android and Blackberry provide.
Short answer: no iPhone 4S is truly unlocked. Unlocked means: usable on any carrier it is capable of supporting.
Unlike most phones, the iPhone 4S can be used on both US-based CDMA and all GSM carriers worldwide. In the US, there are four variants available: one for AT&T, one for Verizon, one for Sprint, and the one Apple sells unlocked. It’s the same hardware for all variants, the only difference being which operator it is locked to, which is done when the phone “phones home” to Apple’s servers.
Apple’s so-called unlocked device can be used on any GSM carrier but cannot be used on either Sprint or Verizon. Likewise, the Sprint and Verizon versions of the iPhone 4S can be unlocked so any foreign GSM SIM can be used, but not a US SIM card (i.e. one from AT&T, T-Mobile, or one of the smaller regional GSM providers).
This is purely a decision made by the two US-based CDMA carriers: Sprint and Verizon. It’s clearly not a technical restriction, and there’s no reason for it other than to force you to buy a new phone needlessly and lock you into another two-year agreement.