Hey, rbob! Generally speaking, I wouldn't recommend straying too far from the Network, APN, and VoLTE settings we have for the Google Nexus 6 on Support. As far as the settings that you have enabled currently, I think those are fine as long as she's not seeing any trouble and you're not in a situation where she'd want the device to connect automatically to WiFi. For example, I could use WiFi at home, but there's a tower about a block from here, so since the network is super strong it makes more sense for me not to bother setting it up with WiFi preferred because then I just don't have to deal with the WiFi dead zone that is our kitchen corner.
As far as the error message she's seeing, is it asking her to log in for a data connection? Are there any verbiage specifics you can give me to search? I'm glad it's not impacting things but I'm happy to do some digging and find an explanation for you guys if it's one we have listed internally!
Sorry to have dropped that question and then wandered off. I tend to visit only when I have a problem, so I've gotten out of the habit.
Executive summary: There's really nothing wrong. I really need to stop fiddling around with this phone.
The login to network sequence is a little more complicated then I remembered.
First, an icon appears in the top status line, a representation of the standard signal strength bars with a question mark. Pulling down from the top to bring up the notifications, I see the same icon, with "Android System" adjacent. Below are two lines of text, "Sign in to network" and "T-Mobile." If I select the message, I'm taken to "u.web2go.com/upsell" which offers links to change the plan.
When I first turned the phone over to my wife, WiFi calling was enabled, but when she tried to make her first test call from home, she was told that WiFi calling wasn't supported by her plan. I just enabled it again, set the phone to "WiFi preferred" and was able to make a call. So, it looks like I either had something else set wrong on my initial setup, or something's changed in the meantime.
So far, the only noticeable side effect to turning data off is an "X" appearing next to the signal strength icon in the status line, and a message at the top of the settings display informing me that it's off.