voluntary vs unpaid

voluntary is a noun but unpaid is not a noun.

Word	Noun	Adjective	Verb	Adverb
voluntary	Yes	Yes	No	No
unpaid	No	Yes	No	No

voluntary (noun)	unpaid (noun)
composition (often improvised) for a solo instrument (especially solo organ) and not a regular part of a religious service or musical performance
(military) a person who freely enlists for service

voluntary (adjective)	unpaid (adjective)
of your own free will or design; done by choice; not forced or compelled	not paid
controlled by individual volition	engaged in as a pastime
	without payment