Technology columnist Walter S. Mossberg answers readers' questions

Q: I'm from Italy. Since the new iPhone 4 is not on sale in Italy yet, I was wondering if it's possible to buy it in the U.S. without a contract and if I would be able to use it with an Italian carrier's plan when I go home.

A: According to AT&T, the answer is essentially no. It's a bit complicated, but AT&T says it offers a 'no commitment' iPhone 4 starting at $599, or $400 more than the same model costs with a two-year contract. For this price, you don't need to sign a contract or pay an early termination fee, though you still need a monthly plan. However, AT&T says all the iPhone models it sells in the U.S., including these no-commitment phones, are locked and cannot be used with other carriers' SIM cards. So, you could almost certainly 'use' it in Italy and pay high AT&T roaming fees, but you couldn't just pop in a SIM card from an Italian carrier and switch to that carrier's plan from AT&T's.

As with prior iPhone models, it might be possible to 'jailbreak,' or modify, the new model to get around this restriction, but doing that can have downsides, including security vulnerabilities and incompatibility with future official software updates.

Q: Is it possible to buy an unlocked iPhone overseas and use it in the U.S. on T-Mobile instead of AT&T?

A: Apple says that, in some countries, though not all, you can buy completely unlocked iPhones that can be mated with any compatible carrier's SIM card. However, if you are referring to the new iPhone 4, note that it uses a new type of much smaller SIM card than most phones, and T-Mobile or whatever other carrier you wanted to use would have to offer such cards.