hot line vs telephone circuit

hot line and telephone circuit both are nouns.

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

As nouns, telephone circuit is a hypernym of hot line; that is, telephone circuit is a word with a broader meaning than hot line:

Other hypernyms of hot line include line, phone line, subscriber line, telephone line.

hot line (noun)	telephone circuit (noun)
a direct telephone line between two officials	a telephone connection
a telefone line that provides direct and prompt information or advice to the caller in relation to a specific subject