I found this for T-Mobile 4G LTE only, is it correct (bold means dominant band)?
T-Mobile 2, 4, 12 1900, 1700 def, 700 a
What about others, such as 3G?
Hey healthylaff! I'm glad you're doing your research! Does this page provide the answers you need: T-Mobile network bands & technologies? If you're thinking about bringing your own device, you may want to check out this space as well: Not Sold by T-Mobile.
Regarding your Wi-Fi calling question, you're correct - you'll need a supported T-Mobile Wi-Fi calling capable device to be able to utilize that functionality. Let me know if you have any other questions!
Iwould suggest checking out this site
You can select T-Mobile and the specific phones you are looking at and see what is supported by that phone and what T-Mobile offers.