answer vs call back

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

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

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

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

answer (noun)	call back (noun)
a nonverbal reaction
the speech act of replying to a question
a statement (either spoken or written) that is made to reply to a question or request or criticism or accusation
a statement that solves a problem or explains how to solve the problem
the principal pleading by the defendant in response to plaintiff's complaint; in criminal law it consists of the defendant's plea of `guilty' or `not guilty' (or nolo contendere); in civil law it must contain denials of all allegations in the plaintiff's complaint that the defendant hopes to controvert and it can contain affirmative defenses or counterclaims

answer (verb)	call back (verb)
give the correct answer or solution to	recall knowledge from memory; have a recollection
respond to a signal	summon to return
understand the meaning of	return or repeat a telephone call
react to a stimulus or command	cause to be returned
react verbally
give a defence or refutation of (a charge) or in (an argument)
be satisfactory for; meet the requirements of or serve the purpose of
match or correspond
be liable or accountable
be sufficient; be adequate, either in quality or quantity