hold on vs call up

hold on is not a noun while call up is a noun.

Word	Noun	Adjective	Verb	Adverb
hold on	No	No	Yes	No
call up	Yes	No	Yes	No

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

hold on (verb)	call up (verb)
be persistent, refuse to stop	recall knowledge from memory; have a recollection
stop and wait, as if awaiting further instructions or developments	get or try to get into communication (with someone) by telephone
hold the phone line open	bring forward for consideration
hold firmly	call to arms; of military personnel
retain possession of	cause to become available for use, either literally or figuratively