I guess I'm really dim. I thought the iPad Air 2 came with a sim card that could be used by any of three carriers: Sprint, T-Mobile, AT&T. Going through Settings, it looked like I could set up right from the iPad (esp. being a T-mobile client), but of course there were password issues. Called T-mobile, they said, "the sim card doesn't start with '840...' , we can't help you. Called Apple, they said go to the T-Mobile store and pay $20 for a new sim. Nevermind, I get it, the card is just a placeholder-- I can use the tablet with any carrier, as long as I fork out for their specific sim.