Is my ex-wife able to block phone calls preventing me from communicating with my children?

It's unlikely that a judge would restrict your ex-wife from blocking your calls. But if there is a custody order in place, you could file a motion to modify custody that includes a provision regarding phone contact with your children. Provisions may include specific days and times during the week when you are entitled to speak with your children by phone. Violation of such an order could lead to contempt charges against your ex-wife.

I'd contact the attorney who handled your divorce to seek an order outlining what kind of contact with your children is required.