Phone-silencing button (4)
I believe the answer is:
mute
I believe this is a double definition.
'phone' is the first definition.
(thesaurus)
'silencing button' is the second definition.
(I've seen this in another clue)
(Other definitions for mute that I've seen before include "Silent, refraining from speaking" , "Soften the sound" , "One refuses to answer" , "Device that softens the tone of a musical instrument" , "Silent - dumb" .)