phone vs hold the line

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

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

phone (noun)	hold the line (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

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