phone line vs subscriber line

phone line and subscriber line both are nouns.

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

As nouns, phone line and subscriber line are synonyms defined as:

Other synonyms of phone line include line, telephone circuit, telephone line.

phone line (noun)	subscriber line (noun)
a telephone connection	a telephone connection