I think it's funny for att or any company to charge more than $250 for a desposit.
The US govt will give up potentially up to a $729k housing loan with as little as 3.5 downpayment or even still 0% down if doing a VA housing loan. And they have low credit requirements. It used to be given to people with credit scores in the 500s. And even with the housing crash, you can still get hundreds of thousands in loans with a credit score in the low 600s.
$600-700 is a ridiculous amount of money for a desposit considering ur monthly bill probably won't be more than $100-125 a month with one line.
Landlords usually don't charge more than one months desposit on their homes.