AT&T will not unlock an iPhone under any circumstances. Together with their SMS pricing this has odd results (including a glut of former iPhones).
Verizon and Sprint claim they'll allow unlocking, but the reality is more complex (emphases mine)...
... What consumers need to understand is that there are actually four different versions of the iPhone 4S: Verizon, Sprint, AT&T, and Apple. Only the Apple phone, available from their stores or on-line, is fully unlocked and can be used on any carrier. The other phones are permanently locked and cannot ever be used on another carrier in the U.S. Even if you spend $800 for an unlocked phone as I did and dedicate it to a single U.S. carrier, you are locked into that carrier forever if you want to keep using the iPhone. Neither Apple or the other carriers will fully unlock your carrier phone.
Apple declined to return phone calls and emails to discuss this matter. However, the high level technical support supervisor that I spoke with at length indicated that she personally thought that customers should be warned about the different versions of the phones and ramifications of buying a phone directly from a carrier rather than Apple. A senior spokesperson for Verizon told me that all Verizon phones are locked to their network and she did not quite understand the problem. I am willing to bet that Verizon customers would precisely understand this issue, even if she did not.
So if you paid retail for your iPhone 4S and purchased it from Verizon, Sprint, AT&T or Best Buy, you will never be able to use that phone with a different carrier. In my view, this is a scam by these companies. I think the cellular providers should tell their customers up front that locked phones really mean that you are locked into the carrier forever. While all of these phones are iPhone 4S with the identical hardware, the difference is that if you purchase your phone directly from Apple, it is truly a universal device and will allow you to change carriers without the penalty of making the phone worthless.
I would urge the Carriers, FCC, or Courts to enforce the following rules to protect cellular customers in the United States: consumers buying new cell phones must be informed of the existence of any SIM LOCK (also known as a network lock) on their phone before sale; wireless phone companies must unlock handsets upon request, without fee, when a consumer purchases a new phone outright (unsubsidized) without a contract; wireless phone companies must unlock handsets upon request, without fee, when a consumer comes to the end of their contract, or at any time thereafter. Carriers must fully unlock their phones (as does Apple with the iPhone 4S) and not partially unlock the phone to block access to other carriers and deny consumers the right to choose any provider anywhere...
Carrier locking of iPhones is a nasty trick on consumers. It reduces the value of a post-contract iPhone; I wonder if a clever lawyer could sue carriers for theft. Another lawyer might find the shared policies a sign of illegal collusion or price-fixing among carriers.
Senator Franken - this is right up your alley!
- Gordon's Notes: The Gordon vs. AT&T iPhone war - Conclusion (11/2011) - we dramatically cut our monthly charges.