I can sympathize with you MM22, because I was the child who was the recipent of 2 such phone calls when I was a kid. Both callers told my father, they do not think that little BPT is a good influence on thier sons. Yes, it hurts real bad.
But under the circumstances you described, consider yourself lucky that your child is protected from someone who says, “I’m going to kill you”. Its not a joke, and while not necessarily a real threat, it does reflect what the child is hearing / seeing at home.
The main thing for you to do now, is keep building your child’s self esteem. With time, he will overcome the obstacles he now faces, and then you / he will have the last laugh.
Oh, and by the way. The two kids for whom I was deemed to evil to play with?
I’ll not list specifics (after all, this is YWN) but suffice to say, they are not a source of nachas for their fathers (or children) right now.