challenge vs call out

challenge is a noun but call out is not a noun.

Word	Noun	Adjective	Verb	Adverb
challenge	Yes	No	Yes	No
call out	No	No	Yes	No

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

challenge (noun)	call out (noun)
a call to engage in a contest or fight
questioning a statement and demanding an explanation
a formal objection to the selection of a particular person as a juror
a demand by a sentry for a password or identification
a demanding or stimulating situation

challenge (verb)	call out (verb)
issue a challenge to	call out loudly, as of names or numbers
take exception to	utter aloud; often with surprise, horror, or joy
raise a formal objection in a court of law	challenge to a duel
ask for identification