If while you're creating the new boundries related to him, you allow him to call the children at a pre-arranged time each week - Saturday mornings? Or whatever works for you both. He would have an opportunity to show you his committment to the family and you would have an opportunity to see how serious he is about the committment by how often he keeps the appointment.
You can't have him emotionally upsetting you whenever he feels like picking up the phone. If you limit his access, you will have plenty of time to work on yourself and your emotions while still letting the kids have access to dad.
There's absolutely nothing wrong with being prepared for a phone call - people do it in business every day.
Give yourself freedom - don't wait around for the phone to ring. You can enjoy your life and still allow him some time with his kids.
CB