Voicemail signal (4)
I believe the answer is:
tone
'voicemail signal' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tone that I've seen before include "Quality of a person's voice" , "Sound - style" , "Sound signal on a telephone" , "Colour quality; note" , "Quality of a musical or vocal sound" .)