I tried a couple of places I had very poor or no receptions on my 5s, deep inside my local Sam's club for one and in my home's basement for another. 2 dots now both places and people who get my calls say voice quality is now excellent where before it was marginal if I could connect at all. As a phone this is great and it the sole reason why I upgraded to the 6s, 5s was otherwise sufficient for my needs.
Data rates are not very good on band 12 so is a good reason t-mobile prefers band 4. I think it is a good strategy on t-mobile's part to use 12 as a backup when 4 is not working well. Use band 12 when needed also frees it up so it is available to everyone when it IS needed.
T-mobile is supposed to be using band 2 as well, but I haven't seen it when I check in field-test mode, just 4 and 12.
FYI: Band 2 (1900 MHz), Band 4 (1700/2100 MHz), Band 12 (700 MHz)