brush up vs call up

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

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

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

Other hypernyms of brush up include call back, recall, recollect, remember, retrieve, think.

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

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