telephone extension vs extension phone

telephone extension and extension phone both are nouns.

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

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

telephone extension and extension phone: an additional telephone set that is connected to the same telephone line

Other synonyms of telephone extension include extension.

telephone extension (noun)	extension phone (noun)
an additional telephone set that is connected to the same telephone line	an additional telephone set that is connected to the same telephone line