I just got a iphone 4 used. I have not used iphone b4. It's a ATT phone, but is appearently unlocked because I did I hard reset to clear it and it still takes my TMO sim, and seems to work fine, except that the hotspot randomly disappears. Sometimes it'll stay on for an hour, sometimes(rarely) it'll stay on indefinitely. Most the time, it shuts off between 30 min and 3 hrs. When I go look at the phone, the hotspot reports to still be on, but all devices on the network have lost it, and it wont reappear until I turn the hotspot off and back on. Then it's fine again for however long... SUPER FRUSTRATING!!! I am subscribed to hotspot service, and currently, at least, I am not over my plan limit for tethering so it "shouldn't" be an account issue... Anyone know anything about this?