call up vs hold on

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

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

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

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