The two bands on the Sprint phone are popular elsewhere in the world - world phone.
I was in Korea, in an area with CDMA coverage with the same exact frequencies we used - even with the radios matching, I had no coverage.
I've been in parts of Asia with AT&T GSM phones where I could call but not get data.
AT&T's model has 3 frequencies rather 2 or 4.
More frequencies = more coverage...
... supposedly, but even when the frequencies match, you don't get guarantees and when they don't - you're stuck.
Closest you can get here is the ATT or T-Mo phone (but so far as I know, that last one is doing its own thing with AWs on one of those channels, so for world coverage) - you'll still max out at 3 with those.
UMTS frequency bands - Wikipedia, the free encyclopedia
PS - you can do voice on UMTS - http://www.umtsworld.com/technology/wcdma.htm