Android 6.0 Marshmallow has been out for a while now, but up until now the Nexus 6 running on T-Mobile’s network has not gotten an upgrade to the latest OS. Today however the company’s product manager has announced that the upgrade has finally been approved and that the roll out will begin soon.
According to Des, the upgrade was held back until now due to ensuring full compatibility with Band 12 LTE and WiFi Calling. While you could have flashed Marshmallow manually anytime you wanted, WiFi calling wouldn’t have worked properly until installing the MRA58X build. The update includes Google’s latest November security patches and the update should be rolling out in the very near future.
— Des (@askdes) November 20, 2015