Yes, many phrasal verbs could be described as idioms in the sense that their meaning is not obvious from the two (or three) words composing them.
But another way of looking at it is to say that a phrasal verb is just another verb, and you need to learn it whole, not try to deduce the meaning from its parts. Under this view, phrasal verbs could not be regarded as idioms.
Defining idioms is quite difficult and there are no hard borders. Even single words are sometimes described as idioms: for example, some dictionaries regard "fishy" (in the sense of "suspicious") as an idiom.