phone line vs telephone line

phone line and telephone line both are nouns.

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

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

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

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