Possible answers:
(1). If you can't determine how much time went by, then it could be a sign that your friend is still busy, or perhaps she is just not that responsive to your calls. Check your friend's phone provider's website.
(2). Your friend may have taken you off her radar. If so, your friend might not call.
(3). Don't wait for calls to be missed. If your call is overdue, it's time to log the call.
(4). If the balance of missed calls, answered calls and placed calls seems off to you, think of other reasons this may be. Perhaps her phone plan is limited or she can't buy phone credit/minutes/top-up regularly.Choose from options above and answer: What most naturally follows?

How to tell if someone is ignoring your calls and decide what to do about it
Check your call log.
Check to see if all your calls to your friend have been missed. What's the ratio of missed calls to answered calls? Notice the call length, the time you called her, how often you call her and whether or not she has called you.
Answer:
(4).