To access T-Mobile's 700 MHz A block spectrum (marketed as "Extended range LTE") device must support LTE band 12.
The following unlocked phones not only support LTE band 12 but all other bands required to be compatible with T-Mobile network (LTE bands 2, 4 and 12, HSPA+ on AWS and 1900 MHz, and GSM on 1900 MHz) however band support is not the only requirement. T-Mobile issued a statement regarding band 12 unlocked phones: All Band 12 LTE devices on our network must support VoLTE, E911. Currently only some unlocked phones support VoLTE and band 12:
Under investigation: the following phones may support VoLTE on T-Mobile network however the manufacturers have not made it clear:
Help us find out the status of VoLTE: post a screenshot or a video in /r/tmobile showing LTE in the status bar while on call using a phone above. Also please post Model number and Baseband version shown in Settings > About.
Other LTE band 12 phones listed below are not compliant. Due to lack of VoLTE support you won't be able to place and receive calls including 911 calls in some buildings and some outdoor areas where 2G GSM and 3G HSPA+ signal (marketed by T-Mobile as "4G") is too weak or non-existent while LTE band 12 is available. You will still have data connection and you can receive and place non-emergency calls via Google Hangouts app (using Google Voice number). To place an emergency call you will have to either move outdoors where 2G or 3G signal is available or disable LTE to make your phone roam on another GSM network.
Mostly compatible unlocked devices
The following devices support all LTE bands used by T-Mobile (2,4,12) but lack HSPA+ (aka 4G) on AWS. Starting from 2012 T-Mobile had been launching HSPA+ on 1900 MHz almost everywhere they have HSPA+ on AWS. In some markets like Indianapolis, Louisville, and Chicago there is no AWS HSPA+ anymore as of 6/20/2015. See the official coverage map for striped "4G" (non-LTE) coverage. In "4G" areas the devices below will drop to 2G.
Some carriers (AT&T, Sprint and US Cellular) sell phones that claim to support band 12 however in reality their phones may not have band 12 enabled. Due to this fact and other various issues with carrier-branded phones this section will only list phones that are confirmed to support band 12. If you confirm please make an announcement post in /r/tmobile (mention /u/sgteq in the comments to send me a notification). Also note that devices sold by carriers are typically SIM-locked (except Verizon devices). Check the unlocking policy with the carrier.
Few of the carrier branded US phones support VoLTE except on the network that sold them so the impaired calling warning applies to all such phones.
Models that are known to support VoLTE on T-Mobile network:
Due to the huge number of phone models worldwide this site does not track devices sold outside of the US. To check band 12 support simply check phone specifications. Note that some sellers and manufacturers use outdated frequency notation (700/850/etc). There are seven LTE bands in the 700-800 MHz range so listing 700 MHz as a supported frequency is incredibly ambiguous. Use reliable sources of phone network specifications that list band numbers.
None of the phones sold outside of the US support VoLTE on T-Mobile network as far as Spectrum Gateway is aware so the impaired calling warning applies to all such phones. If you learn about an international phone that supports T-Mobile VoLTE please make an announcement post in /r/tmobile (mention /u/sgteq in the comments to send me a notification).
Notable devices released in 2014-2016 that do not support LTE band 12
Moto E 4G LTE (2nd Gen) XT1527 was released with band 12 but after a few months Motorola disabled band 12 with 5.1 update. Moto G (3rd Gen) model XT1540 was initially listed with band 12 support but Motorola removed the band a few days later. See this article for more details.
In January 2016 Alcatel removed band 12 from the specifications of Alcatel Idol 3 and released a software update that disabled band 12 in all Idol 3 phones. Alcatel support provided a statement to one of customers regarding the update. Contact Alcatel to request compensation.
Last updated 6/14/2016.