Phalanx uses high speed gun to protect specific (small area) target from incoming cruise (not ballistic) missiles. Iron Dome uses rockets to protect large areas from incoming short range ballistic missiles. The two system's capabilities don't overlap. Phalanx could not be used to do what Iron Dome does, and Iron Dome could not be used to do what Phalanx does.
from the article: Iron Dome makes world history intercepting nine Gaza rockets