This phone appears to have the necessary radios to handle T-Mobile 4G at 1700MHz (makes sense since it's marketed as a T-Mobile 4G phone). But it also is indicated as having the ability to operate on 850MHz and 1900MHz, which are AT&T 3G frequencies.
Does this mean that an unlocked Exhibit II 4G could be used on either T-Mobile or AT&T? That would be kind of nice.
Actually it can't get 3G with HSPA+ at all even on T-Mobile, just 4G with WCDMA or Edge with GSM. I can still get 3G here on my old phone but this one only gets 4G or Edge, there's no option in between for 3G.
I don't know about AT&T but using a spectrum analyzer I've found that it works on 1700Mhz using WCDMA to get what T-Mobile calls "4G" but speeds are only up to 5.6Mb at the T-Mobile store where you'd think the speed would be the best. It gets Edge using GSM on 1900Mhz but can't get 3G at all even though they have 3G on 1700Mh here also using HSPA+. I doubt that it gets 3G on AT&T either but it would depend on which protocol they use I expect. I don't suggest buying one unless you are just going to use it on Wi-Fi since they don't have 4G hardly anywhere.