The data speeds on me and my wife's phones were unstable so I decided to check the 'mobile network' setting, and switched the network mode from CDMA/GSM to LTE/CDMA/GSM. Afterwards, both our data connections were much faster, and we now have HD voice during calls. Also the signal icon now shows '4G LTE' as opposed to just '4G'. Not sure why T-Mobile has LTE turned off by default, but you might wanna update your settings if you've got good LTE coverage in your area.