NEW YORK (TheStreet) -- Apple (AAPL) - Get Report  on Tuesday started selling iPhones without SIM cards or pre-determined monthly service plans from a U.S. carrier. It's a boon for iPhone users who prefer not being tied down to any one particular carrier or do a lot of travelling abroad.

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Apple started offering a new, unlocked iPhone model through official retail channels on its Web site and in stores. The SIM-free purchase option now allows users to "activate and use the unlocked iPhone on the supported wireless network of your choice, such as AT&T (T) - Get Report , T-Mobile (TMUS) - Get Report , Verizon (VZ) - Get Report , or Sprint (S) - Get Report in the United States." In the recent past, there was only one unlocked model sold by T-Mobile and made for use on its GSM cellular network.

Apple shares were down 0.56% to $105.65 in early trading Tuesday in New York.

Apple explains that the new, SIM-free option is perfect if "you don't want a multiyear service contract, or if you prefer to use a local carrier when traveling abroad."

Although the unlocked iPhones can be used with all four major carriers, there are differences. Apple warns "if you choose T-Mobile, your iPhone comes with a T-Mobile SIM card already installed" and you need to contact T-Mobile or visit an Apple retail store to activate the handset.

Otherwise, unlocked iPhones (the iPhone 6, 6 Plus, 5S or 5c) do not come with a nano-SIM card and you need to obtain one from any supported carrier. After inserting the SIM, the set-up procedure consists of turning on the phone and following the on-screen instructions.

Apple warns that purchasing an unlocked iPhone means you won't qualify for the lower iPhone price associated with a contract or a carrier installment plan.

Prices for a new, unlocked, SIM-free iPhone 6 range from $649 for a 16 GB model to $749 for a 64 GB model; the 128 GB goes for an eye-opening $849. Add $100 to each of those prices for an unlocked iPhone 6 Plus. Apple estimates all SIM-free phones purchased online will ship within three to five business days as compared to one day for carrier-locked models.

-- Written by Gary Krakow in New York.

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Gary Krakow is TheStreet.com's Senior Technology Correspondent

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.