extension phone vs telephone extension

extension phone and telephone extension both are nouns.

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

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

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

Other synonyms of extension phone include extension.

extension phone (noun)	telephone extension (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