A list of valid phone numbers isn't needed. Calling every phone in a desired area code -- or even every phone number in the North American Numbering Plan (NANP)

A list of valid phone numbers isn't needed. Calling every phone in a desired area code -- or even every phone number in the North American Numbering Plan (NANP) -- is perfectly feasible for a robo-caller. My back-of-the-envelope calculation suggests that phoning numbers in sequence (or chosen at random) will get you a real person on every 6th call. Briefly, my calculation is 293 area codes x 7.9M possible numbers per area code (not 10M because some number formats aren't allowed) divided by 360M (population of the NANP countries, principally US and Canada, and assuming 1 phone per person) implies that every 6th number is assigned.