call back vs callback

call back is not a noun while callback is a noun.

Word	Noun	Adjective	Verb	Adverb
call back	No	No	Yes	No
callback	Yes	No	No	No

call back (noun)	callback (noun)
	a request by the manufacturer of a defective product to return the product (as for replacement or repair)

call back (verb)	callback (verb)
recall knowledge from memory; have a recollection
summon to return
return or repeat a telephone call
cause to be returned