land line vs phone line

land line and phone line both are nouns.

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

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

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

land line (noun)	phone line (noun)
a telephone line that travels over terrestrial circuits	a telephone connection