I already had ATT when I got the n900, and it works fine. Just plugged in the sim card. Works with voice and data with out any effort from me. Of course, ATT's 3g doesn't work with the N900. Only Tmobile's 3g does in the US market. If your ATT connection is only giving you voice and no data, then either your plan doesn't include data or ATT has fukked something up. I'd call them.