WATS vs phone line

WATS and phone line both are nouns.

Word	Noun	Adjective	Verb	Adverb
WATS	Yes	No	No	No
phone line	Yes	No	No	No

As nouns, phone line is a hypernym of WATS; that is, phone line is a word with a broader meaning than WATS:

WATS: a telephone line; long distance service at fixed rates for fixed zones; an acronym for wide area telephone service
phone line: a telephone connection

Other hypernyms of WATS include line, subscriber line, telephone circuit, telephone line.

WATS (noun)	phone line (noun)
a telephone line; long distance service at fixed rates for fixed zones; an acronym for wide area telephone service	a telephone connection