One of the most interesting wrinkles in the iPhone 4S—which I review at length elsewhere—is its status as a “world phone.” Unlike previous generations, it supports both CDMA (Verizon, Sprint) and GSM (AT&T, the rest of the world) wireless technology right out of the box. But there are some interesting quirks to this product that may please savvy international travelers.

When Apple released the Verizon iPhone 4 in February, the iPhone 4's product line got a little more complicated. Because Verizon’s network in the U.S. uses a different wireless standard (CDMA) than AT&T does (GSM), Apple had to build different chipsets for each network, and thus, different phones. But the iPhone 4S changes all that: It can access either the CDMA or GSM network, depending on which carrier you sign up for.

<snip>

But the iPhone 4S supports both GSM and CDMA networks, and has a micro-SIM card slot no matter which carrier you subscribe to. So if you buy a phone from a CDMA carrier that doesn't use SIM cards, what’s in that slot? The answer is something that’s called a “roaming SIM.” If you're subscribed to a CDMA carrier in the states, when you travel internationally, your device can now switch to an international GSM network using a micro-SIM card. In other words, when outside the U.S., you can still get phone calls to your regular number and connect to the Internet, thanks to a micro-SIM that connects to Sprint and Verizon’s partner wireless networks in those countries—all at much higher rates than in the U.S., of course.

<snip>

But there’s a new wrinkle that potentially makes the international-roaming experience better on Sprint and Verizon iPhones than it is on AT&T. Sprint plans to sell the iPhone 4S with its micro-SIM slot unlocked; Verizon’s will be initially locked, but if you’ve been a customer in good standing for 60 days, you can call Verizon and ask for an “international unlock.” (A Verizon spokesperson told me that this is Verizon’s standard policy for all world phones—it’s just the first time it’s manifested itself on an iPhone.)