employment vs call-back

employment and call-back both are nouns.

Word	Noun	Adjective	Verb	Adverb
employment	Yes	No	No	No
call-back	Yes	No	No	No

As nouns, call-back is a hyponym of employment; that is, call-back is a word with a more specific, narrower meaning than employment:

Other hyponyms of employment include shape-up, booking, reservation.

employment (noun)	call-back (noun)
the occupation for which you are paid	the recall of an employee after a layoff
the act of using	a return call
the act of giving someone a job
the state of being employed or having a job