When it comes to network advancements and improvements, for T-Mobile the buzz is all about VoLTE and their band 12. VoLTE or Voice over LTE should bring about plenty of improvements to the whole of network stability and performance, but not all phones have it, and without support for VoLTE there is no support for Band 12 on T-Mobile's network. Last month, T-Mobile began requesting that OEMs start removing Band 12 support from their devices if they haven't been certified with support for T-Mobile's VoLTE technology, and even though both of Google's latest Nexus smartphones, the Nexus 5X and the Nexus 6P, list support for Band 12 LTE, T-Mobile has seemingly confirmed on their Twitter account that neither phone will have Band 12 LTE support on the network.
T-Mobile's reasons here are not exactly the most simplistic, but breaking it down according to T-Mobile's previous explanation about devices with no VoLTE support, phones that have Band 12 support need to support VoLTE due to T-Mobile having specific coverage areas which don't have any other Band covering the region but Band 12. Without support for VoLTE, any phones in this area are said to not have the capability to place voice calls, so naturally it becomes understandable for T-Mobile to be worried about phones which don't support its VoLTE standard as it could cause issues with customers trying to make calls in those areas.
At the moment, there has been no official confirmation from T-Mobile on this matter, but some responses via Twitter to other users' questions seem to suggest such a scenario. If this is true, it seems a little odd that Google wouldn't have been able to ensure VoLTE support for T-Mobile for either new device. As the certification can be costly, this is one reason why OEM's not quite the size of companies like Google might decide to scrap Band 12 support altogether, but Google can certainly afford these certification fees so perhaps the timing of it all was a bigger factor. T-Mobile also stated in their Twitter response that while the phones apparently won't support Band 12, this is presumed to be just for the launch as T-Mobile mentions they will be continuing to work with Google to enable Band 12 in the future. Having said that, subscribers of the carrier may be able to look forward to Band 12 support down the line if this is no misunderstanding on the status of both devices in this regard.