they use autodialers. they pick an npa-nxx for example 718-338- and have the computer dial sequentially 718-338-0000, 718-338-0001, 0002, 0003 etc… the computers can do this rather quickly. If the number is invalid the computer can detect the network signaling that indicates that and eliminates it from the database. If the network signaling indicates a busy signal, ringing with no answer or the jackpot an answer these numbers are marked. Telemarketers who are covered by the DNC list are supposed to make sure they always have the most updated version of the list and that those numbers are loaded into their database as do not calls. Of course we know how that goes.
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