Some great points, but I disagree with #4. I don’t know what you mean by “another time”.
You know if your boss tends to come in and out all day long, or if he is likely gone for the day, or if he’s just in the men’s room. I don’t want to phone you every half hour until I reach him. Tell me when to phone back. (Or take a message.)
Say “I don’t know when Mr Smith will be back, but you can try again in (15 minutes/one hour/this afternoon/tomorrow).”