land line vs subscriber line

land line and subscriber line both are nouns.

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

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

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

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