Credit score to buy an Iphone 4G?

What’s the minimum qualifying credit score for buying an iphone 4g without a security deposit, which ranges i guess form $100 to $1500?

Is having a Social Security Number (SSN) for a year and paying all my credit cards payments on time, paying over 4 utilities’ bills every month on time, and paying my university’s tuition in full on time enough to establish good credit score that qualifies me to sign a two-year agreement with AT&T?

P.S. I have had an iphone 2g with AT&T for over 2.5 years under my friend’s SSN, does that help me in being qualified for the new iphone somehow?

If I have a cosigner who has an excellent credit score, does that help me also in being qualified and waive the security deposit?

Your answers are welcome and appreciated as long as they answer all of the questions above clearly and with no guesses.
Off-topic answers are unwelcome and will be flagged.

Many thanks.