On the one hand, if he's ending the conversation because emotions are too fired up, that's understandable, though he should do it in a different manner, hanging up on someone is a way of slapping someone in the face or shutting them off, so to speak. It IS rude. You're not wrong about that. If he's the type to come back to the disagreement later, then it's just resolving how he "leaves the argument" that needs to happen.

I wouldn't say that it's a sign of a relationship ending, unless this is new behavior and some negative things have been happening along with your seeing that in other ways the relationship is deteriorating. I just wouldn't jump to that conclusion.

If these arguments never get revisited in a calmer manner than I think he is avoiding important matters and If you can't get him to come to the table on these things, then I would take a good long hard look at your relationship. Arguments, no matter how you look at them are confrontations resulting from a difference in values and opinions and if you can't discuss, come to an agreement or compromise because of this, how can you expect your relationship to grow?