hold the line vs phone

hold the line is not a noun while phone is a noun.

Word	Noun	Adjective	Verb	Adverb
hold the line	No	No	Yes	No
phone	Yes	No	Yes	No

hold the line (noun)	phone (noun)
	electronic equipment that converts sound into electrical signals that can be transmitted over distances and then converts received signals back into sounds
	electro-acoustic transducer for converting electric signals into sounds; it is held over or inserted into the ear
	(phonetics) an individual sound unit of speech without concern as to whether or not it is a phoneme of some language

hold the line (verb)	phone (verb)
hold the phone line open	get or try to get into communication (with someone) by telephone
hold the line on prices; keep the price of something constant