The flip side of this is to consider the point of view of an "innocent bystander". Does someone being angry at an unrelated event hurt them? Does the person describing that event to them, angrily, hurt them, either?
I think that I would trust someone more who had a range of emotion based on what has happened to them recently, than someone who always tried to show a particular "face" to everyone, regardless of their current circumstances.
In that sense, always hiding your anger is actually hurting your chances of being trusted by others, which I think is one of the most valuable things other people can give you.
Normally functioning adults do have some control over their emotions. If one is red-hot angry, they can take a deep breath, count to ten, put the situation into perspective, and reflect on the feelings of others. It is not an accident that this is what we tell children to do when they're having a fit-- it fucking works! This is not "hiding" anger, it is recognizing and dealing with anger.
Or, perhaps they managed to reduce their base anger response, and aren't actually hiding anything?
It could also mean that they are constantly trying to game people into believing they feel a certain way but they actually don't.
Or it could mean they were taught to do this at a young age, and it's an automatic response.
In any case, it makes one look less human and more likely a manipulator, or at least concerned with the manipulation of people enough to suppress an automatic reaction.
I doubt that many would agree with you that those who react more calmly "look less human". People are generally drawn to those with a calm, positive demeanor, as they are often enjoyable to be around.
Everyone trying to hide their anger all the time leads to fake, sterile interactions, and pent up hostility, in my opinion.
We'd be a lot healthier of a society if we didn't project and expect a perfect image of everyone all the time.
For example, in the hypothetical phone call, what if you say to John:
"John, I want to say I'm feeling very frustrated and angry right now. I expected a short call and I had to wait 35 minutes instead. I realize it's not your fault, but I wanted to get the feeling out, because you would have noticed it in my voice anyway."
Also, yes I do believe that someone in the presence of someone else being angry should not automatically assume it is about them.
Certainly, there are times when anger is justified and appropriate, but it is helpful to properly control it and not to aim it at people who are innocent bystanders.