T-Mobile uses Band 4 (Primary), Band 12 (New-Primary), and Band 2 (Rural areas)
Most likely with T-Mobile, you'll be on B4 90% of the time. B12 is still being deployed, and even then you only connect to it if you can't get B4 signal (building penetration)
Verizon is Band 13 (Primary), Band 4 (Secondary), and Band 2 (Rural areas)
B13 is pretty congested for Verizon, so a lot of them are being forwarded over to B4 when they can.
AT&T is Band 17 (Primary), Band 4 (Secondary), Band 2 (Secondary), Band 5 (Rural), Band 29 (New-Secondary), Band 30 (New-Primary).
AT&T's B17 is pretty congested as well, so they're trying to get people on B4/B2 when they can. Also, B29 is a download-only link (with a similar penetration as B17), and that's not in all areas either. B30 is new, a bit higher frequency (2300), but would help in dense areas.
The way AT&T and VZW started off LTE, they used low-band (700b/c)(B17/B13). This had a great advantage w/ building penetration (the lower the more penetration). T-Mobile started with a higher band B4 (1700/2100), good for speed, but not for long-distances or building penetration. However, AT&T and VZW devices are so saturated with 700 that their speeds are suffering, hence why they are trying to alleviate load with the other bands (primarily B2/B4). AT&T interestingly has a lot of bands to juggle as well, with B29 (700d) for download, and B30 (2300) for new-primary.
T-Mobile finally got in the low-band game most recently with B12 (700a), but still has a bit of catching up to do.
So technically a B2/B4 phone works on all 3 carriers (AT&T/VZW/T-Mobile), although B2/B4 is only secondary for AT&T/VZW. T-Mobile benefits the most since B4 is T-Mo's primary. If you're in a large metropolitan city (probably top 50 cities in the US), AT&T and VZW probably deployed B2/B4 to help alleviate the congestion on their B13/B17 bands, so most likely you'd connect to B2/B4 if you had AT&T/VZW in these large cities.
