Links that talk about T-Mobile asking manufacturers to disable band 12, what band 12 means and what are the supported devices etc.
On phones that dont support band 12, on testing found that during a phone call the speed drastically reduces to 2G. This behaviour is true on ATT too depending on where you are and the quality of coverage of the carrier.
T-Mobile on Lumia 635 when not in call data speed is 60+Mbps down, 14Mbps up; During a call 3Mbps down 0.2Mbps up 🙂
T-Mobile on iPhone 6s nothing changes, when not in call and during call 90+Mbps down and 20+Mbps up!