Just so anyone who is wondering knows, you can load the OTA update to LYZ28J. This is the latest 5.1.1 build for T-Mobile that enables WiFi calling and includes the Stagefright security fix. It worked on my new Nexus 6.
I wanted to take advantage of the sale price for a generic US model, which would be exactly the same as one purchased direct from Google or Motorola. But I wanted to get WiFi calling like my wife's Nexus 6 that came from T-Mobile. I called both T-Mobile and Motorola to ask: If I put a T-Mobile SIM in the phone, would it then update to the T-Mobile OS version? Couldn't find an answer to that online anywhere.
Both answered no it would not. One T-Mobile guy said it might, but not to count on it. Another T-Mobile guy said definitely not because the Nexus 6 T-Mobile sold had some special hardware added. Yeah, sure it does. Motorola said not gonna happen. I bought the phone anyway, hoping they were wrong and with a gut feeling they were.
They were wrong. I put an unactivated T-Mobile SIM in and during initial setup when it asked if I wanted to install system updates I postponed it so I could get the SIM activated first. After activation I started installing system updates. It did three and when it finished, sure enough I had LYZ28J and the option to enable WiFi calling!
So it found that it was on T-Mobile and automatically installed the correct version. Very nice! I cannot say if this would work on a Nexus 6 that was already fully updated on another network. Would changing SIMs prompt a change in the OS? My feeling is it wouldn't. But if you first reverted to an earlier version to trip a system update, then I'm sure that with an activated T-Mobile SIM it would install the T-Mobile OTA build.
Now there is absolutely no difference between my wife's T-Mobile supplied Nexus 6 and my Amazon supplied one. Except for the price we paid.