land line vs landline

land line and landline both are nouns.

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

As nouns, land line and landline are synonyms defined as:

land line and landline: a telephone line that travels over terrestrial circuits

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