There are 2 major carriers in the US built on GSM technology: AT&T and T-Mobile USA

http://en.wikipedia.org/wiki/Cellula..._Frequency_Use

The "basic" frequencies are as follows:

GSM (2G)
AT&T - 1900, and 850 in some markets
T-Mobile - 1900

3G & 4G
AT&T - 1900, and 850 in some markets
T-Mobile - AWS (1700/2100), which is different from the 2100 band used in Europe; 1900 in addition to/instead of AWS in some markets

It is safest to assume that the bands cited above are the "baseline" assumptions of what should work for each carrier. But, if you look at the chart (and at other resources), it gets more complicated than that:

- There are some limited roaming agreements in place that allow T-Mobile customers roaming privileges (for voice only) on AT&T's 850 network in some places where T-mobile has no coverage

- AT&T does not have 850 available in all markets. But you'll ideally want a module with both 850 and 1900, since AT&T will "load-balance" between the two where they have both bands, to help manage capacity

- In general, T-Mobile seems to be moving it's 4G service to it's AWS frequencies while repurposing the 1900 band as primarily 3G. Or else, build new 3G coverage either via AWS or 1900, but not both. You already see T-Mobile 3G on the 1900 band in a lot of markets, but it is certainly not universal. So, if you are a T-mobile subscriber, you might still be able to get 3G with the PHS8 variants (except for PHS8-E) if you are in an area where they have added 3G to the 1900 band, but there are no guarantees.

- AT&T and T-Mobile seem to be developing LTE on the 700 band as well

- In theory, you may be able to use the variant that includes CMDA (PXS8) with Verizon (the largest CDMA carrier in the US). But Verizon, as of right now, does not provide SIM cards for unlocked phones for the purposes of accessing it's CDMA network.

So, net, it's really complicated. No variant will perfectly meet every need, especially if you want to use high speed data both in the US and abroad.