Another way to look at it is that there lies one of the reasons your husband and his ex are no longer married. If you want it to stop, you and your husband have to set clear boundaries. However, you only have control of your own behavior. Your husband chooses not to set that boundary with his ex...for whatever reason it is that he chooses not to.
If it bothers you, don't answer the phone when you see that she is calling. Let her leave a message. If she calls repeatedly, tell her not to do that anymore, and that once she leaves a message, you or your husband will get back to her when you are able.
Boundaries. It is very hard for some people to respect, and it is very hard for some people to enforce.