The voice on the line? (4)
I believe the answer is:
bass
I believe this clue is a double definition.
'the voice' is the first definition.
(bass is a kind of voice)
'the line?' is the second definition.
(bass is a kind of line)
'on' acts as a link.
(Other definitions for bass that I've seen before include "Very low - perch" , "Type of fish" , "Sea fish; voice" , "Fish; vocalist" , "Fish - low voice" .)