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.AT&T can & will charge a deposit for adding a line. Adding a line requires a credit check.