This is a question beyond our scope.
Our scope is sexual health and relationship. What you are talking about is mental health. earlier medical health and mental health were the prominant branches, later on taking some from both, sexual health was another branch created. What you are asking us is on mental or psychological health.
A simple example, a rape. What damage that occured on the body is health issue, what damage it occured on sexual impact is another issue, and what it did on psychological factor is another issue. Though its a rape, still some might get very big trauma, some might even enjoy the force act.
You are asking us due to phone and voice how she is suffering. Can we tell that? It is not possible, some might take that very sportingly, some might feel real fun, some might find as a business opportunity, and in reality it might be some obsessed lover giving a message. If she is mentally stong, wont even care, but if not, then she will keep calling for professional help by consulting psychiatrist. You are a good friend, the only reason she can tell this to you is when she feels other people are giving proposals but you are yet to give any... its just a message to you that she likes you.
Two small suggestion- ask her to consult police, they have cyber crime unit, can track the phone or even if from IP based too. And second suggestion is, not to pick any and all number. these days a lot of people lose their mobile balance by picking these phone or making call back to these suspicious numbers.