Apple began accepting orders for the unlocked version of the iPhone 4S on its United States online store.

The 16 GB version retails at $649, the 32 GB retails at $749.00, and the 64G GB retails at $849.00.

The unlocked version is not compatible with all networks.

“The unlocked iPhone works only on supported GSM networks, such as AT&T in the U.S.,” said Apple on its website. “When you travel internationally, you can also use a micro-SIM card from a local GSM carrier.”

The company also added that it would not work with CDMA carriers, such as Verizon Wireless or Sprint.

The unlocked version of the iPhone 4S is useful for those traveling abroad, because they can gain access to the device by inserting a local carriers SIM card overseas.

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Sprint also began SIM-locking all iPhone 4S models bought from the carrier on Nov. 11 and onwards.

“…All iPhone 4S devices will have the SIM locked. The locking occurs during the activation process and is invisible to the customer,” according to SprintFeed.

Apple stores were selling contract-free iPhones in mid-October, according to 9to5Mac.

All GSM smartphones sold off contract were unlocked, and they will stay that way despite the iOS 5.0.1 update.

Sprint offers a specific plan for customers with SIM-locked iPhones.

“Customers can sign up for one of our international rate plans and use this phone is most countries all over the world,” Sprint Feed added. “When traveling internationally, there is a setting that must be turned on within the device to connect to GSM.”

The unlocked iPhone 4S was announced for a November release late last month.