landline vs telephone line

landline and telephone line both are nouns.

Word	Noun	Adjective	Verb	Adverb
landline	Yes	No	No	No
telephone line	Yes	No	No	No

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

Other hypernyms of landline include line, phone line, subscriber line, telephone circuit.

landline (noun)	telephone line (noun)
a telephone line that travels over terrestrial circuits	a telephone connection
	the wire that carries telegraph and telephone signals