phone system vs telephone system

phone system and telephone system both are nouns.

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

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

phone system and telephone system: a communication system that transmits sound between distant points

phone system (noun)	telephone system (noun)
a communication system that transmits sound between distant points	a communication system that transmits sound between distant points