subscriber line vs telephone circuit

subscriber line and telephone circuit both are nouns.

Word	Noun	Adjective	Verb	Adverb
subscriber line	Yes	No	No	No
telephone circuit	Yes	No	No	No

As nouns, subscriber line and telephone circuit are synonyms defined as:

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

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