My iPhone keeps connecting to another carrier (T Mobile)

I have an iPhone 4S from Softbank (Japan), but I'm currently in the US visiting family. Even though I turn off cellular data and roaming, my iPhone automatically connects to T Mobile. This makes me nervous because I just want to use the wifi at my family's house. I don't want to be hit with a massive phone bill. Does anybody know what's up? Any help would be great!