If you left a message the first time calling her and she hasnt called back in a day or two, Call her again. DONT LEAVE ANOTHER MESSAGE. I wouldnt call more than one other time if you havent spoken to her before.
After calling twice and leaving a message once, You have pretty much exhausted what you can do without looking like a fool. A playful text maybe.
Phone calls between guys and girls who dont know each other, like my man above said, should be brief. Always ask if shes busy beforehand, dont be rude - you could be catching her at a bad time. If she says shes busy and will call you back in a little bit, tell her you are going to be busy doing (whatever) then but you will call her in a bit. Always keep the ball in your court cause even the nicest girls are prone to not calling back. Remember, she doesnt know you too well, and she most likely isnt trying to pull you, so you will be remembering to call her alot more than her remembering shit.
So again, dont be on the phone for more than a few minutes. If its going great, its an even greater time to end it. I only talk as much as I need till I know shes in a good mood and then ask for what I want. Dont ask her out on a specific date, just be like:
"well I have some stuff to take care of so im gonna let you go, but you and I should get together sometime"
This is truly a vague statement, but it works and it lets her know that you are the man. Dont ask her, tell her. no questions, be a man.
shes gonna say yes, because you havent given her something to say no to. If she says "what do you have in mind?" I just say something completely retarded like "I was thinking I would take you to the park and impress you with my hopscotch skills". I get a laugh, then I tell her "alright ill speak to you later." This gives you a day or so to set up whatever you think will work, and implies that you will call her again. She will be waiting. text in between days, something funny with a

at the end, so she knows you are out there.
This all can be done in 5 minutes. Phone calls arent the place to woo a girl. You make it short, be nice, be funny if you can, and vaguely TELL HER(dont ask) that yall should do something sometime.
any constructive criticism is appreciated.