beeper

noun

1. a device that connects into a telephone circuit and transmits a periodic signal as an indication that the conversation over the circuit is being recorded.
2. a similar device attached to a free-ranging animal as an aid to learning its habits; biotelemeter.
3. Also called pager. a pocket-size electronic device whose signal notifies a person of an important message, sometimes displaying the telephone number to be called.

n.

“device that emits beeps,” 1946, agent noun from beep (v.).