Soon after the announcements of Google's 2015 Nexus phones, it came to light that both of them would have band 12 on T-Mobile's network disabled at launch pending certification from the carrier. It now appears that the wait is over, at least for the Nexus 6P.
T-Mobile has now added Google's larger 2015 flagship to its list of devices compatible with band 12. What we don't know, however, is when owners will get the required software update to enable the functionality. Considering that band 12 support is now official, it probably won't be very long. It's very possible that Google could simply roll it out with the December security update. That's purely speculation, however. Time will tell.
In case you've missed all the drama surrounding band 12 on T-Mobile, the carrier has asked OEMs to disable band 12 support in their phones pending a certification process. This is due to several issues that could arise. The long and short of it is that there are a number of places on T-Mobile's network where you may have a band 12 signal and nothing else, because 700 MHz (band 12) reaches twice as far as 1,700 MHz, where T-Mobile has their traditional LTE as well as the HSPA/GSM fallback. Since a lot of unlocked phones don't support VoLTE yet, you could be in a situation where your phone shows an LTE signal for data, but you'd be unable to make a phone call due to the lack of VoLTE, and not being in range of a 1,700 MHz signal for fallback.
Since the Nexus 6P has been certified now, hopefully it won't be much longer before we see a similar green light given to the Nexus 5X.
T-Mobile has clarified that the certification process for band 12 on the Nexus 6P is ongoing and said certification is not yet compete. Read more here.