reply vs call back

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

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

As verbs, call back is a hyponym of reply; that is, call back is a word with a more specific, narrower meaning than reply:

Other hyponyms of reply include counter, field, sass, come back, rejoin, repay, retort, return, riposte, bridle.

reply (noun)	call back (noun)
the speech act of continuing a conversational exchange
a statement (either spoken or written) that is made to reply to a question or request or criticism or accusation

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