That interesting.
I'm on a Sprint plan, with a Sprint phone and T-Mobile SIM. My phone LOVES to stay on Sprint B25/B26 while in my basement, even though I can get much stronger B/N 71 or B12...or even B2 T-Mobile. Sometimes even on my main floor, where I definitely get much strong T-Mobile signals. Looks like they are still using the Sprint network very much here.