What next?

Well if you feel up to it, ask her out. The phone is personal but only to a certain level. Ask her on a date and see what she says. If you're not ready for a date, then continue talking on the phone until you are ready.

I remember talking on the phone for hours when I was your age, I really enjoyed all you could learn about the person that way. Keep up the good work.

Is this a girl you see at school? If so why don't you talk to her at school and offer to walk her home and then make your move Romeo. Don't listen to these chumps, the phone is for girls talking to other girls or else your going to stay in the friend stage 2yrs buddy?
Listen when you see her physically depending on how smooth you are you can make a date out of it without having to ask her out smooth operator

In my opinion I say wait like a day or 2 then call her back, if she calls you back within that 2 day period well its all good it shows she has interest if not oh well don't sweat it, but more importantly how long have you known this girl? If its been for awhile then ease into asking her out if not then give it some time