I've been told if VoLTE is not enabled, band 12 is deactivated. Hence the issue with X3. Sounds like I'll likely have same issue with 950XL unless I can activate VoLTE. If not a simple fix, I'm ready to throw in the towel again.
Getting the 950 XL to use T-mobile VoLTE and IMS services is a hack. It is not completely easy, but it can be done. That phone was never T-mobile branded otherwise we would have a ROM for it and it make things 10 time easier. But there are other Lumias that do have a T-mobile ROM available with proper files/provisioning. Hence is why an option even exists to try and do this on other Lumias.
I am no expert on this but VoLTE is used for Voice/Phone calls over 4G LTE. It supports better call quality and more data. I am not certain who told you that band 12 gets deactivated on a phone if VoLTE is not enabled but that sounds incorrect. Not used maybe while VoLTE is disabled, then ya maybe...
If I turn off/disable IMS (VoLTE provisioning) on the Lumia 950 XL then I can still use band 12 for data or voice and it is still enabled. Just not getting that benefit of the 4G LTE protocol to have HD Voice and VoLTE...VoLTE being enabled or not should have no affect on whether an LTE band is active on the phone or not. The phone either supports/has an LTE band or not. The VoLTE part for T-mobile from what I know is provisioned on the phone end. T-mobile might have a way of determining which handsets support their VoLTE protocol or not (Whitelist) but from what I understand from Windows Phones is that as long as the phone is able to have the basic foundations available in the OS for IMS/VoLTE support on T-Mobile then it can be activated. There is obviously much more to it than this for each handset. I do know the basics for how the newer Lumias worked on carrier networks using Windows 10 mobile were pretty much all the same. The Lumia 650, 950 and 950 XL I have used all three. Using the 650 and 950 XL on T-Mobile and the 950 has been used too each with VoLTE HD Voice working. I have never owned a Lumia 550.
The IDOL 4s and HP Elite X3 share the same basic SOC SD820 MSM8996. But for some reason HP did not hard code the ability for it to work on carrier networks the way the IDOL 4s does. I have not figured out exactly why or what it is. But I kind of have an idea. The same provisioning the IDOL 4s uses for T-mobile and AT&T VoLTE and HD Voice does not work for the HP Elite X3. At the NVRAM level where all the options lie for Cellular and Telephony capabilities the HP is either limited not enabled or not configurable for some reason unlike other handsets that are.