hailer

noun

1. A person who or a device which calls, summons loudly, or hails.

   “"Hullo, Downe—is that you?" said the driver of the vehicle, a young man of pale and refined appearance. "Jump up here with me, and ride down to your door." The other turned a plump, cheery, rather self-indulgent face over his shoulder towards the hailer. "O, good evening, Mr. Barnet—thanks," he said.”