The words “Microphone muted” are adjacent to the “mute” icon. However, the words are in a shaded area that is clearly connected with the “hang up” icon. The principles of proximity and closure compete.
When actively engaged (sorry) in a phone conversation, it’s easy to hang up inadvertently. At least, it was easy for me.
I believe Skype has now fixed this by a redesign that eliminates the conflict.