Band 12 is the 700 MHz block A that T-Mobile won over a year ago. 700 MHz being a lower frequency has a longer range (quite a bit more complicated but that's the layman's explanation). LTE is still supported on bands 2 and 4 and they have no plans on changing that as far as I can tell. They're trying to use multiple bands at once currently for faster speeds in the future. The greater coverage of T-Mobile's LTE in the past 18 months or so is due in a large part to the acquisition of band 12.