telephone line vs phone line

telephone line and phone line both are nouns.

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

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

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

telephone line (noun)	phone line (noun)
a telephone connection	a telephone connection
the wire that carries telegraph and telephone signals