call back vs reply

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

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

As verbs, reply is a hypernym of call back; that is, reply is a word with a broader meaning than call back:

Other hypernyms of call back include answer, respond.

call back (noun)	reply (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

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