It depends on whether the plan is prepaid or postpaid. Postpaid plans are the ones which establish a line of credit since the phone company is tracking your tab and asking you to pay it down at the end of each month.

With that being said, phone companies want as many people as possible on prepaid plans because why wouldn't they want to borrow your money every month? If you want postpaid you need to specifically ask for it and also pass a credit check.