When you broke up, you established a boundary
He is violating your boundary.
He does not understand boundaries.
Tell him it is over and you don't want him calling.
If he continues to call anyway, tell him again to stop calling.
One thing not to do is not answer the phone and expecting
him to figure out that means don't call me. You must
tell him over and over until he gets it.
Eventually you can get legal help if necessary -
if so, come back and ask about a restraining order and
procedure for getting it.