First off, we have to remember that Elly has an almost pathological need to over-extend herself because she thinks others expect it of her. In this instance, it takes the form of assuming that she is required to drop everything and answer the phone owing to the misapprehension that the person at the other end of the line will think less of her if she doesn't answer it. In her mind, they don't notice all the times when she answers the phone but will hold a missed call against her forever.
Secondly, we also have to bear in mind that Elly always, always assumes the worst in any given situation. This means that the missed call could have come from the police or a hospital telling her that a loved one is either injured or dead or from a potential employer who would only offer her a job the one time.
Third, we have to consider the fact that Elly suffers from the belief that her family is trying to keep her from talking in the first place. The kids aren't monopolizing the phone for the same reason she did when she was young, oh, my, NO!! They aren't talking to their friends, they're trying to keep their poooooooor mother silent because they haaaaate her and looooooooove CHAOS!!!!
Fourth and finally, Elly loves to run her God-damn mouth about nothing at all and begrudges her children any opportunity to express themselves. In her mind, they can only speak on the phone when they're paying for it.
Notice that nowhere in the process is there any room for reason, empathy or intelligence. None of those three things are Elly's standard policy.