There are a few variants with different model numbers, and while they all will work for calls and 2G data with any GSM carrier in the world, you'll have to match up the right model to your carrier for 3G. For example, the model we reviewed here C6603 works with AT&T's 3G. Model C6602, which Sony sells direct from their website in the US, has 3G for both AT&T and T-Mobile US (the two largest US GSM carriers). The Xperia Z won't work on the CDMA networks used by Sprint and Verizon.
-------------------- Lisa Gade Editor in Chief, MobileTechReview