Nowadays they offer financing for devices. But even in the past it was required. They would determine the maximum number of lines you had available, and if there were any deposits to open new lines of service. Even before phone financing, those phone contracts came with hundreds of dollars of phone discounts at time of purchase and had hundreds of dollars worth of early termination fees and they want to make sure their customers had a good chance of paying if they left.
Credit checks.
Nowadays they offer financing for devices. But even in the past it was required. They would determine the maximum number of lines you had available, and if there were any deposits to open new lines of service. Even before phone financing, those phone contracts came with hundreds of dollars of phone discounts at time of purchase and had hundreds of dollars worth of early termination fees and they want to make sure their customers had a good chance of paying if they left.