How to unlock your iPhone 4S for world travel

— -- Question of the week: How can I use my iPhone in another country without paying roaming fees?

Answer: The iPhone has all the hardware you'd need to do that, in the form of its Subscriber Identity Module card slot. But this SIM slot — on the iPhone 4 and 4S, it's a smaller micro-SIM — is electronically locked on almost all versions sold in the U.S. That limits you to the $1-and-up international-access charges of AT&T, Sprint and Verizon Wireless.

AT&T doesn't unlock the iPhone, period — unlike other smartphones it sells. But you can buy an unlocked version of the AT&T iPhone direct from Apple, starting at $375 for an unlocked 3GS, $549 for an 8 GB iPhone 4 and $649 for a 16 GB iPhone 4S. (Those high prices reflect the absence of the usual subsidy baked into the iPhone's price, something carriers more than recoup over a two-year contract.)

On this model, you need to choose one of AT&T's voice-plus-data bundles in the U.S., said publicist Mari Melguizo, but can then swap in another carrier's SIM card for your overseas travel.

(Pre-paid plans are widely available and cheap; you can expect to see shops selling them in the airport you arrive in, maybe even before you clear customs.)

Sprint will unlock the micro-SIM slot on its iPhone 4S for subscribers who have been "in good standing" for 90 days, said Michelle Mermelstein, a spokeswoman with the company. You can then pop in any international SIM — but not one provided by a U.S. carrier. The company enforces this by having the iPhone check the country-code and network-code data stamped into every SIM card.

Verizon has more complex rules for unlocking its version of the 4S. Your account must have been in good shape for the past 60 days, said publicists Melanie Ortel and Sherri Cunningham; as at Sprint, you can't use a domestic SIM on an unlocked iPhone 4S.

But Verizon also requires that you complete the unlocking procedure — after getting the new SIM card — with an extra online-activation step in which the phone essentially gets permission from Verizon to use that card. This reminder came from its VZWSupport Twitter account, which pointed me to a customer's report in its tech-support forum.

This added requirement compounds the difficulty of using a Verizon iPhone overseas — you can't pick up any pre-paid SIM once you get there and have it work right away. Forbes writer Marc Weber Tobias worked around this by using the Wi-Fi connection in an Apple Store in London; ZDNet's Matthew Miller bought an international SIM in the U.S. before leaving.

The worst aspect of this mess: Neither AT&T, Sprint or Verizon document these rules anywhere on their own support sites. You either have to ask their customer-support reps directly or have somebody else do the work of piecing together their policies for you. (You're welcome.)

Tip: An app to play DVDs from overseas

If you use the money you saved on international-roaming fees to pick up some DVDs overseas, you'll have a nasty surprise when you get home: They most likely won't play in your DVD player or computer, thanks to the non-U.S. " region codes" embedded in them.