Im trying to get a new cell phone

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I'm trying to get a new cell phone. I turn 18 on Monday and I would really like to get a new phone through T mobile. The only problem is, I dont have a credit card and I need someone who does to buy it and then Ill pay them back. Well I decided to ask my aunt and at first she said it was fine, but now she's not so sure because when I called customer care they said that after the phone is transferred over to me they would have to do a credit check on me, and that there might a deposit that I might have to pay. And she wont keep the phone in her name and let me pay the bill that way because she's being stupid about it. What I want to know is : what is this deposit they're talking about and how much would it be? If anyone could help me, i would really appreciate it. Thanks