telephone circuit vs subscriber line

telephone circuit and subscriber line both are nouns.

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

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

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

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