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&T as well.
Best is to double check with T-Mobile.
Answer from Ruskes on Stack ExchangeVideos
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&T as well.
Best is to double check with T-Mobile.
Apple published detailed pages about both iPad LTE Support and iPhone LTE Support. These pages have a breakdown on device model versus carrier coverage for current (iPhone 6 and 6 Plus, iPad Air 2 and iPad Mini 3) and old devices.
From this table you can see carrier support per device number.
Group 1 (USA):
iPhone 6 Model A1549 (GSM) and Model A1549 (CDMA)
iPhone 6 Plus Model A1522 (GSM) and Model A1522 (CDMA)
Aio, Alaska Communications, AT&T, Consumer Cellular, Cricket, Family Mobile, GCI, Net 10, Straight Talk, T-Mobile, Union Wireless, Verizon
Group 2 (USA):
iPhone 6 Model A1586 (GSM) and Model A1586 (CDMA)
iPhone 6 Plus Model A1524 (GSM) and Model A1524 (CDMA)
Appalachian Wireless, Bluegrass Cellular, C Spire, Cellcom, Chariton Valley, Chat Mobility, Defense Mobile, MobileNation, Nex-Tech, NorthwestCell, nTelos, Pioneer Cellular, Sprint, STRATA Networks, Syringa, Thumb Cellular, United Wireless, US Cellular
I find awkward that iPhone 6 on the second group has the same list of LTE bands as the iPad Air 2 listing, but the list of international carriers is shorter. This can be caused by either iPad LTE page being more up to date, or because this is actually dependent on carrier configuration and contracts with Apple.