let's reverse the plot a bit.
say you "MISTAKENLY get a call from your ex fiancé on whatsapp. It didn't end the best of ways, and you haven't been in touch in any form since"; as far as you'd be concerned, the 'fact' that it was a mistake would be irrelevant. not only would this damage your current standings with your current partner (as insecure partners exist. let's not kid ourselves here. this is a reality of life that's ever more present as the days go on); it would come off as pathetic, creepy and desperate. you'd judge him for all eternity (if you had the same viewpoint as he did; that exes are meant to stay that way) for doing so, and call him a pathetic low-life for not moving on - a situation that should Never happen (however it does based on how you conduct your life - you love to keep in contact with your ex).
what you fail to remember/take note off is, that just because he's an ex; it doesn't entitle you to judge his manner of interacting with you. it doesn't allow you to belittle or question his way of dealing with the fact that you're not together anymore. it's over. it's done. calling him would just add insult to injury and split open a wound that shouldn't have been opened in the first place (speaking from his supposed viewpoint in which i can only hazard a guess as to the reason for his appropriate conduct).
we as humans possess the ability to Choose how we want to live our life and expect to be given the chance to do so without judgement (unless illegal/what-have-you). you likely didn't see him commenting on your exes when you were together, and if this is his way of dealing with the break-up; why can't you allow him to do so in peace?)
at some point in time, it should occur to you, that not everyone sees the world the same way that you do; they don't deserve to be judged for that