1900 MHz UMTS is not enough

Phones marked with the icon on the right do not support AWS (1700/2100 MHz, band 4) UMTS/HSPA+ band. As of 2017 it's necessary to have both AWS HSPA+ and PCS (1900 MHz) UMTS support for a reliable voice calling if your phone does not support VoLTE. 

Starting from 2017 T-Mobile has been shutting down PCS UMTS network in various markets in order to use the spectrum for LTE. As a result T-Mobile 3G voice network uses either AWS or PCS UMTS in various places. The remaining thin 1900 MHz 2G GSM network is not good enough for reliable voice calling in the markets where PCS UMTS has been shut down.

850 MHz UMTS/HSPA+ support is required to access full coverage provided by domestic roaming partners.

References:
  1. T-Mobile shutting off HSPA+ service on its AWS spectrum market by market
  2. Network upgrades from U1900 to LTE