WATS vs subscriber line

WATS and subscriber line both are nouns.

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

As nouns, subscriber line is a hypernym of WATS; that is, subscriber 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
subscriber line: a telephone connection

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

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