A call note changes suddenly (3,2,4)
I believe the answer is:
all at once
'suddenly' is the definition.
(I've seen this in another clue)
'a call note changes' is the wordplay.
'changes' 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 "Simultaneously - immediately" , "Suddenly" , "Together" , "Without warning" , "How daffodils appeared to Wordsworth" .)