Why do AT&T, Verizon and Sprint need a deposit?

Because I went to any AT&T couple weeks ago to get the iPhone 4 and the guy told me I needed a 300 dollar deposit, why do you need it

I have never heard of such.

To get a contract phone, which means you get a $599+ phone for only $199 or less with a 2yr contract agreement, Wireless carriers want to know if you can pay the bill ON TIME and with NO HASSLE and if you will be a GOOD CUSTOMER to them… Just like banks, car dealers, furniture outlets, etc etc etc… They use your FICO CREDIT SCORE, which is imprinted on your Social Security Number for the rest of your life…

$300 deposit tells me that either 1) you have zero to little credit history… Or… 2) you have some credit history, but your late paying your monthly bills or have some bills left unpaid…

if you pay the deposit, and are a great customer for 12 months straight, that means no late payments, no suspensions, no cancelations, you will get your $300 back on the 13th month of contract

Sure it wasn't the phone fee? Depending on the phone you choose, some don't require that you pay for the phone up front because they're cheap. Phones like the iPhone are more expensive and they require a couple hundred up front and you pay the rest with your two year contract. I paid $100 up front for my phone when I got a plan with ATT. Its just to be sure they don't get screwed out of money. If you don't pay that up front and you cancel your account immediately after opening it, then they basically would have given you a phone for free.