I agree with you about the tradeoffs and the importance of looking at the problem from all angles, but I would argues that this is clearly the case wher aesthetics won over usability, which should _never_ happen. It should be easy to visually separate the "hang up" button from other, much less destructive and more commonly uses buttons (especially "mute"), for example with some small amount of separating space. The only reason this is not done is that it "wouldn't look balanced" and designers would be unhappy.

In my experience designers are the worst people to hire for UX because they often sacrifice usability for aesthetics. Programmers fare a bit better (usable UI usually doesn't conflict with the code quality), still not perfect though. Casual users are probably best, with some education in usability of course.