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AT&T can & will charge a deposit for adding a line. Adding a line requires a credit check.
As long as you've paid your bill on time, you shouldn't have to pay anything for the deposit. When i started with At&t (i was 20), i just left Sprint and since i paid my Sprint bill on time all of the time, i went from paying a $500 deposit to nothing. And a 20 year old definitely doesn't have any other reason on his credit besides that to having the deposit drop like that.
 
It let me upgrade even though I wasn't eligible

I bought the 3Gs on its release day last summer. With a 2-year contract, that should put my upgrade date at next summer, right? I just tried upgrading on the AT&T site and it let my order for an iPhone 4 go through at the $299 price. Did I get away with something or did it work the way it was supposed to?
 
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