He sounds utterly hateful. Keep a diary of the abusive phone calls and any other unwanted contact. He sounds angry at you - was it your decision to separate?
Try not to let the children know what an arse their dad is being, and in time, when everything has calmed down, he may re-establish a reasonable relationship with them.
Meanwhile, write yourself a list of all the new opportunities you can take advantage of without such a toxic presence in your life, and the fun you and your children are going to have.
Take care