The factory image which includes Wi-Fi calling on T-Mobile has now been made available from Google for the Nexus 6.
Android 5.1.1 build LYZ28E has just been made available from Google for Nexus 6 owners who have T-Mobile to download and install. This release brings both the factory image and OTA update files, allowing users to update their devices without having to wait. One of the big feature additions in this build was Wi-Fi calling for T-Mobile customers. The update began pushing out to a select number of users this past weekend, but now anyone can install it.
Installation of these files is a little bit different than just accepting an OTA update on your device. You can find out how to use these files below.
Source: Nexus Factory Images