Hey Hotaru, I got this girl's number, called her and she's frequently unavailable, what should I do?
Quit calling, and move on.
She's avoiding you. For one reason or another. Don't blame yourself. But don't call her anymore.
If I call a girl, and she can't make a little date and doesn't arrange time for one at a better juncture....well....
It's a sign. Heed it and carry on.