telephone system vs central

telephone system and central both are nouns.

Word	Noun	Adjective	Verb	Adverb
telephone system	Yes	No	No	No
central	Yes	Yes	No	No

telephone system (noun)	central (noun)
a communication system that transmits sound between distant points	a workplace that serves as a telecommunications facility where lines from telephones can be connected together to permit communication

telephone system (adjective)	central (adjective)
	in or near a center or constituting a center; the inner area
	serving as an essential component