Is the iPhone 6 model A1549 (CDMA) compatible with T-Mobile?
The GSM side of Verizon phones are unlocked and will work on domestic
  carriers but there is no guarantee all features will work. It is not
  the same for Sprint, the GSM side of that phone is unlocked and
  unlocking it will only allow it to work with non-domestic GSM
  carriers.


A1549 (CDMA): 


  This is considered as the world phone version. It supports all the
  same spectrum frequencies as the A1549 (GSM) and also includes CDMA
  EV-DO Rev. A and Rev. B (with 800, 1700/2100, 1900 and 2100 MHz
  bands). Besides, it works on UMTS/HSPA+/DC-HSDPA (850, 900, 1700/2100,
  1900, 2100 MHz), GSM/EDGE (850, 900, 1800, 1900 MHz) and 4G LTE bands
  include bands 1, 2, 3, 4, 5, 7, 8, 13, 17, 18, 19, 20, 25, 26, 28, 29.
  The model is compatible with Verizon and Cricket in the US. However,
  if you buy an unlocked Verizon phone, you will be able to use it on
  any other GSM networks like T-Mobile and AT&amp;T as well.


Best is to double check with T-Mobile.