HTC One M8 red triangle and exclamation mark

[AC Question]

in history/story a triangle with an exclamation mark appears next to an incoming call. What does this mean? I know a missed incoming call is just an exclamation mark. Is it because the phone was turned off to reject the call?

 

[the tall guy]

Hi, it could be - it sounds like the call was rejected or didn't make it through to the ringer.