Luckily, in a large number of areas, band 12 has been allowed to be deployed. Here's a coverage map:
https://www.google.com/maps/d/u/0/viewer?mid=zYsWXH8QR36Q.k_csKbu63eVk. It gets at probably the biggest metros in the US.
While band 17 does overlap with band 12, the phone's baseband radio software has to have band 12 support built into it for proper hand-off of voice support. TMo requires phones with band 12 to also support VoLTE because band 12 is being used to expand rural coverage where there is no existing footprint. If a phone doesn't support VoLTE in this situation (no legacy coverage), the phone will have data but not voice or messaging.
So TMo actually finds itself in a bit of a unique situation. It's also the reason the Nexus 6 has its own baseband radio compared to all the other variants.