Simple Mobile is an MNVO of T-mobile (Simple Mobile sells service on T-mobile's network and rebrands it as Simple Mobile... That could be why your Nexus S is displaying T-mobile as your service provider. I hope this helps you.
Sent from my Llama powered N1 courtesy of the xda app! Llama Power!!
Samsung Galaxy Tab 10.1 WiFi | Samsung Galaxy S3 T-Mobile Slim 4.2.2.build 6 | Samsung Galaxy S4 T-Mobile Slim build 6
Ladies and Gentlemen, Xzibit: "Yo dawg. I heard you like being robbed, so I put a fee on yo fees so we can fee you while we fee you.