Let me see, if I understand your question right. Let's say, his name is Markus Huber. If you don't know him, you will call him "Herr Huber". And you will continue doing so, unless one of you two offers to change from "Sie" (formal you) to "du" (informal you). Then you will start calling him Markus. Germans can only call each other's first name, after they agreed to use "du". It is possible between friends or colleagues to only use the last name and just call him "Huber", but I would advise against doing so, because it could come off as rude. If you work in a German office and everybody just calls him "Huber" you can also do that, but best is to always wait and see, how the Germans call each other in a certain context. For the switch from "Sie" to "du" it is also best to wait for the German native speakers to come forward. Even for Germans it is tricky to decide, how to address each other.

Hi Dora. Same as in English, really. Depending on the relationship and situation, you either use the first name or "Herr H....." (Mister) / "Frau H...." (Miss, Misses). Using the last name without Herr or Frau is something you only here in the military. When the counterpart wants you to use his first name, they will kindly ask you after the first introduction.

From my opinion the German prople are really quick to call you by your first name and to tell you to do the same. Even elderly people, who are approx. 40 years older then you. They say things like, "My parents gave me a name and I want to be called by my (first) name." You'll find only a few people who want to be called by their last name, especially if you know them already. If you call someone by thier last name or say "sie" instead of du, you show them that you want to keep a distance to them. Often it is NOT rude to say the first name. At least that's my experience. :) Hope that helped.