Apple starts selling unlocked iPhones in U.S.

Apple is selling "unlocked" iPhones in the U.S. for the first time, allowing owners to switch carriers to a limited extent.

The unlocked iPhone 4s are listed Tuesday on Apple Inc.'s website for $649 and $749. They're identical to the version sold for use on AT&T Inc.'s network, but don't require a two-year contract.

The buyer supplies his own Subscriber Identity Module, a card that ties the phone to a network. Apart from AT&T, the only national U.S. carrier that's compatible with the phone is T-Mobile USA, and it can provide only phone calls and low data speeds.

Many overseas carriers are compatible with the phone, so international travelers can switch out their U.S. SIM card with one from the local country to avoid AT&T's international roaming fees.