Having a friend who does not return phone calls can be a worrying situation. A person might automatically think the worst in such a situation. This would mean that you might, for example, think that you had done something to annoy your friend. Maybe she is not answering because she's mad at you.
But there are other things that could be going on that would mean that she has a different motive. First of all, maybe she has lost her phone. This would mean that she does not even know that you are calling. Second, maybe she is in some situation where returning your calls would be inappropriate. Maybe she's at church, or in a class, or doing something with her family. In any of these cases, her motive is that she does not want to act in a way that is inappropriate for the situation she is in.