Ok - speaking from experience, I would have to say, women love to chat and yes on the phone. It is not based on confidence as it is just a voice call. They speak with those they choose to, and yes my friend she chooses not to with you.
She may be in a relationship and can not talk in front of the other, who knows, who cares. If it has been a few weeks already of getting to know one another and she still does not want to, then you need to come to the real understanding.
We have all been there, including me. Yes it is confusing, yes it boggles the mind, yes we can not stop thinking about it.
So to avoid future interruptions to your heart, move on. Just say it nicely, that you need to have a friendship with a person other than a text.