telephone circuit vs telephone line

telephone circuit and telephone line both are nouns.

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

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

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

telephone circuit (noun)	telephone line (noun)
a telephone connection	a telephone connection
	the wire that carries telegraph and telephone signals