If you're a T-Mobile user, likely the first spec you look for on an unlocked device is to see if it supports Band 12 LTE, otherwise known as 700MHz. The reason for this is it's the only low-band spectrum that T-Mobile has. It's also what T-Mobile is using to build their future, essentially. As they are running VoLTE over the 700MHz band. And that is the main reason why many smartphones that are not sold by T-Mobile are coming without support for that Band. Now T-Mobile and Google had stated when the Nexus 6P and Nexus 5X were announced that they were working to get Band 12 support on both devices. It looks like we're almost there now. As T-Mobile now has the Nexus 6P listed on their page of Band 12 supported devices.
For those of you looking for a new smartphone, however, this page is a great resource for picking the right phone for T-Mobile's network. Now that T-Mobile has added the Nexus 6P to their list of Extended Range LTE devices, it shouldn't be long before we see Band 12 LTE added to the Nexus 6P. It should be as simple as a software update, which could come with the December security patch which should be coming out in the next couple of weeks. In fact, we know it can be done with an over the air update, because Motorola is updating the Moto X Pure Edition to have Band 12 LTE support with the Marshmallow update. So there's that for those on T-Mobile looking for a vanilla Android device. Two great choices at great prices.
What's interesting to note is that the LG Nexus 5X is not listed on the page. So it looks like T-Mobile and Google still have a bit of work left to do with this year's Nexus devices in getting them to be fully compatible with T-Mobile's network. The reason for Band 12 LTE being such an issue is that it requires some extra certification, since it is also the band that phone calls will be made over and are currently being made over. So T-Mobile wants to be sure it's being done correctly.