call back vs brush up

call back and brush up both are verbs.

Word	Noun	Adjective	Verb	Adverb
call back	No	No	Yes	No
brush up	No	No	Yes	No

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

Other hyponyms of call back include know, recognise, recognize, refresh, review.

call back (verb)	brush up (verb)
recall knowledge from memory; have a recollection	bring to a highly developed, finished, or refined state
summon to return	refresh one's memory
return or repeat a telephone call
cause to be returned