trunk line vs telephone circuit

trunk line and telephone circuit both are nouns.

Word	Noun	Adjective	Verb	Adverb
trunk line	Yes	No	No	No
telephone circuit	Yes	No	No	No

As nouns, telephone circuit is a hypernym of trunk line; that is, telephone circuit is a word with a broader meaning than trunk line:

Other hypernyms of trunk line include line, phone line, subscriber line, telephone line.

trunk line (noun)	telephone circuit (noun)
a telephone line connecting two exchanges directly	a telephone connection
line that is the main route on a railway