Just because it has Band 12 doesn't mean it works on T-Mobile's spectrum. I had the single SIM 950 and it works on T-Mobile of course, but it's because it does so on Band 4. You need to have VoLTE as well as E911. As taken from T-Mo's support page:
On T-Mobile, any device on its network with band 12 LTE data support must also support T-Mobile Voice over LTE services and E911.
Devices compatible with Extended Range LTE - Spectrum Gateway
Read above, I posted about this.
It has the correct spectrum/frequency support, which I explicitly tested for, on 12. Really it has support for almost any carrier in the US, except Verizon, and probably most overseas couriers too, it's got a really wide-range built-in.
I got close to 50mbps, in this test, using ONLY 12.
VoLTE isn't a factor in any of this, it's a way to enable voice/text, over a pure LTE data stream. No US carriers support this for the 950XL, currently, they only support branded Windows phones, unfortunately (for now, hopefully this will change).
Edit: I see why the VoLTE statement figures in, and *maybe* it's a factor, but I'm suspicious. This site isn't a T-Mobile one, and I don't know how much of this data is verified?
Anyway, while I got screaming fast data on band 12, with my 950XL, I don't know if I tried voice (not LTE obviously) when I was experimenting. So, maybe, possibly, it's a factor, if T-Mobile ONLY allows data (the VoLTE part) on 12.
Edit#2: I found a more "familiar" link:
http://www.tested.com/tech/564027-h...-more-control-over-your-unlocked-phone-draft/
Anyway, if this is right, then voice on Band12 won't be possible on T-Mobile, which is a pretty bum deal, since that's where almost all their expansion is, and it seems where it's planned as well.
Currently, T-Mobile isn't allowing VoLTE on any non-branded Windows phones, in the US. Hence, they are effectively "locking out" any devices that implement the band, where people want to BYOD.
They're becoming reasonably friendly to non-branded devices for Android, but apparently Windows just gets ditched.
That said, fatclue_98 I apologize, I see now why the LTE IS a factor, if an odd one. And I also see why I won't be switching back to T-Mobile anytime soon, given this policy/implementation.
How bizarre, I really didn't expect to figure this out tonight, digging around.
All of the US carriers suck, now it's just a matter of figuring out which is least bad, for your locale. Good !#@% luck ;-]