I can definitely see how this would be frustrating.
To me this is like leaving the room and slamming the door behind you. Sometimes it needs to be done to prevent boiling over and saying/doing something worse. Some people just need a cooling off point before they can go forward.
It was obviously a hot topic so maybe setting up an appointment to discuss such-and-such would prepare each party in advance so they could go in with a cool head.
Maybe you can develop a joint strategy where the other says "I need to cool off" or "we need to set up a time to talk about this" or some other key phrase so the other one knows this is a hot topic. Something to avoid phone hang ups or walking away...
Slamming phones and doors is a defense mechanism for people who feel overwhelmed by the discussion or haven't had adequate time to think about their feelings on it. They become overwhelmed and react. Some people just need space and introspection before they can continue in a civilized conversation.
Sorry, this probably isn't what you wanted to hear but this is what came to mind when I read your post.
Aren't relationships fun!?