People or machines that dial phone numbers, or devices that automatically call phone numbers in sequence.
From dial (to call a number on a telephone) + -er (one who does this action). The verb 'dial' comes from the rotary dial mechanism on old telephones. Plural is formed with -s.
Before smartphones, a 'dialer' was often a telemarketer or robocall machine—so this word has become almost synonymous with annoying automated calls, which is why 'dialer' has picked up negative connotations.
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