call up vs brush up

call up is a noun but brush up is not a noun.

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

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

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

call up (noun)	brush up (noun)
an order to report for military duty

call up (verb)	brush up (verb)
recall knowledge from memory; have a recollection	bring to a highly developed, finished, or refined state
get or try to get into communication (with someone) by telephone	refresh one's memory
bring forward for consideration
call to arms; of military personnel
cause to become available for use, either literally or figuratively