centrex vs telephone exchange

centrex and telephone exchange both are nouns.

Word	Noun	Adjective	Verb	Adverb
centrex	Yes	No	No	No
telephone exchange	Yes	No	No	No

As nouns, telephone exchange is a hypernym of centrex; that is, telephone exchange is a word with a broader meaning than centrex:

centrex: (CENTRal EXchange) a kind of telephone exchange
telephone exchange: a workplace that serves as a telecommunications facility where lines from telephones can be connected together to permit communication

Other hypernyms of centrex include central, exchange.

centrex (noun)	telephone exchange (noun)
(CENTRal EXchange) a kind of telephone exchange	a workplace that serves as a telecommunications facility where lines from telephones can be connected together to permit communication