WATS line vs subscriber line

WATS line and subscriber line both are nouns.

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

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

WATS line: 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 line include line, phone line, telephone circuit, telephone line.

WATS line (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