Please help me?

You could go down the old fashioned route of sending her a letter!  At least then she can choose when to read it (a bit like a text), plus she can take her time, replying.

So WHY is she choosing to ignore your phone calls? There is surely more to this than her being "a massive introvert." 

First of all, introverts NEVER or rarely, answer the phone. It doesn't matter how important the phone call might be, you should never expect them to pick up or ring back even if you desperately need them to. Because they won't. Introverts are ridiculously self centred people, they are only out for themselves and what they can get. They don't care how their behaviour makes other people feel, all they care about is what benefits them. 

Sorry to be so blunt but it's true. They are really hard work and tbh the fact your friendship with this girl is broken is a blessing in disguise. Since you say texting is out of the question (did she block you or something?) I think maybe an audio message, email or even facetime might be the best thing to do in this situation. Also, if it was something you did that broke the friendship, have you tried apologising? 

Leave her be? If she's ignoring calls, I'm not sure what other options you see here. Presumably she knows how to reach you and is aware you've tried to reach her. Sometimes we have to simply respect someone's desire to be left alone.

Go visit her and take her out for lunch and ask her why she going down the deep hole and that you care for her mental health