trunk line vs phone line

trunk line and phone line both are nouns.

Word	Noun	Adjective	Verb	Adverb
trunk line	Yes	No	No	No
phone line	Yes	No	No	No

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

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

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