T-Mobile is slowly expanding coverage of sites using the low-band 700MHz (Band 12) LTE spectrum in the United States. The good news is that Sony Xperia Z3 owners will be able to take advantage of this. T-Mobile has confirmed that the Xperia Z3 will get 700 MHz support in May 2015 via a software update.
Not all areas will get 700 MHz coverage as T-Mobile does not hold a license for all places and in other regions TV stations occupy that spectrum. Still it is good news nonetheless that support is being added, if you’re lucky enough to be covered.