The iPhone 4S features Siri, a virtual voice-activated assistant not available on other iOS devices. Photo: Jim Merithew/Wired.com

If you’ve been craving an iPhone 4S, but you travel a lot, or don’t want to be tied down by two-year data plan, you can grab your very own unlocked, contract-free model starting today.

One caveat, however: The unlocked model is only available on GSM networks, such as AT&T or global providers, not on CDMA networks like Verizon or Sprint. Apple is careful to note this on the purchase page of the Apple Store’s website.

For T-Mobile users, that means, yes, you can...