A couple of days ago it was discovered that T-Mobile was requesting certain OEMs to remove band 12 LTE support from their unlocked devices. This was something of a big thing for T-Mobile since band 12 LTE is their low-band spectrum which they have never had. Band 12 is important for building penetration as well as a more expansive coverage so naturally T-Mobile would likely want to keep as many of their customers supported with band 12 LTE as possible. So, why ask to have it removed from certain phones? At the time it wasn't completely known why this was happening but new discoveries that have come to light as of today give a little bit of an explanation as to the reasons behind T-mobile's actions.
According to the details, the removal has a little bit to do with the VoLTE and E911 support. Some devices may not support VoLTE but do have support for band 12, and here is where the issue lies. If an unlocked device, for example lets say the Motorola Moto E, doesn't support VoLTE and is connected to the network via band 12 LTE, E911 calls would not be possible in the event that the device wouldn't be able to connect to another band on the network. This could cause a major issue for T-Mobile since E911 support is a requirement for phones which are sold in the U.S., and the only way to seemingly correct this issue is to make sure that the phone supports VoLTE. If not that then the only other way to essentially make a device gain network connection would be to remove band 12 support so that the device would know to try and connect to another band.
Supporting VoLTE on band 12 would be possible for OEM's should they submit their devices which don't support it to be put through T-Mobile's network certification process. This could potentially cause another issue as there is no clear explanation of what might happen (if anything) should any particular OEM decide they don't want to go through with the certification process which as it happens can not only be costly but has to be done for each device individually. With the possibility of more costs, OEMs could simply choose to forego the certification and just remove band 12 support altogether. Right now there is no way of knowing whether the devices which fit into this category will remove band 12 support or choose to go through the certification to gain VoLTE support. T-Mobile also didn't give any explanation of how devices may be affected if band 12 support would be removed, Would they take those devices off the network and no longer offer them? Would they leave the devices as is without support for band 12 if the OEMs choose to remove it? It's all unclear at the moment.