In the upper left of my iPhone 6 (iOS 11.0.3), I was surprised to see "T-Zero" instead of "AT&T". I was in the Messages app when I saw this. It changed back to "AT&T" after I pressed the home button to return to the home screen.
So I googled "t-zero network on my iphone" and the search results reminded me that I have an app called "t-zero" installed on my phone. I use it all the time but I had forgotten it was called that. I opened it to see if the network would change but it didn't.
Did the T-Zero app change my network? Can apps do that? Or is it an iOS glitch?