Suddenly a call note is changed (3,2,4)
I believe the answer is:
all at once
'suddenly' is the definition.
(I've seen this before)
'a call note is changed' is the wordplay.
'is changed' indicates anagramming the letters.
'call'+'note'='callnote'
'callnote' anagrammed gives 'llatonce'.
'a'+'llatonce'='ALL AT ONCE'
(Other definitions for all at once that I've seen before include "Together" , "Suddenly" , "Simultaneously - immediately" , "Without warning" , "in unison" .)