telephone line vs telephone circuit

telephone line and telephone circuit both are nouns.

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

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

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

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