Although more smartphone manufacturers are going down the path of selling directly to consumers rather than through carriers, a move welcomed by many consumers, there are some pitfalls to be on the lookout for. One of those concerns band 12 LTE and use of these phones with T-Mobile. Following similar issues for the Motorola Moto X Pure Edition, T-Mobile indicates they have managed to get Google to disable band 12 in the new Nexus 5X and Nexus 6P, at least until Google gets the devices certified for VoLTE.
Sources indicate the heart of the problem appears to be connected to areas where T-Mobile only has band 12 available. In those places, to place any kind of call including emergency calls, a device has to support VoLTE. Unfortunately, T-Mobile does not appear to be able to switch a device over to roaming in those situations.
Smaller manufacturers are unlikely to pursue certification due to the cost and time investment. Some think Google could do this, but despite their deep pockets, sales of Nexus devices is likely a very small contributor to their bottom line that does not justify the attempt.
source: Android Police
