telephone circuit vs phone line

telephone circuit and phone line both are nouns.

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

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

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

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