Buzzing existed on phone (4)
I believe the answer is:
whir
'buzzing' is the definition.
(I've seen this in another clue)
'existed on phone' is the wordplay.
'existed' becomes 'were' (I've seen this before).
'on phone' indicates a 'sounds like' (homophone) clue (how the word might sound over the phone).
'were' sounds like 'WHIR'.
(Other definitions for whir that I've seen before include "Spin with a humming sound" , "Rotate with hum" , "Make a humming noise" , "Prolonged buzz" , "Soft swishing sound" .)