I know there are different levels of complexity of messages within any animal species communication system, especially among individuals that are part of a social, family or feeding/hunting group. Individual state of mind, location of food, danger, locating other family/social group members, etc. are all possibilities.

Dogs in a neighborhood are related in species but not necessarily in any formal hierarchy or commonly cooperative group. Certainly individuals recognize each other by voice and I suspect the calling out of those are answering one another is a form of "I hear you and I am out here also." A strange dog's voice can certainly elicit more attention from the regular neighborhood dog population by virtue of individuals working to identify this new voice and state their own presence and territory.

On a lighter side I always enjoyed the Gary Larson cartoon showing the scientist that invented the animal translator device finding out what dogs are really saying. As he walked through a neighborhood dogs barking in all locations are saying, "Hey, hey, hey, hey."

I have had marvelous close relationships with members of different species of animals over my life but for me, now, there is no substitute for being greeted or communicated to in my own language. When I go downstairs in the morning I hear "Hellooooo" and from a different location a bright "Good Morning!".