callback vs call back

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

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

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

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