All right, so you're winding up the call. There's a light at the end of the tunnel. But you need an exit strategy, and also a way to maintain the momentum you've built throughout the conversation. If your goal for the call didn't include asking her out, then you both might end up feeling unsure of where things stand once you hang up.
So, make a plan. It doesn't have to be huge. Tell her you'd like to call her again tomorrow night, or maybe bring up the football game this weekend. Is she going? You can meet up at halftime for a cup of hot chocolate. It can even be as simple as saying, "So, hey, I guess I'll see you in class tomorrow. Talk to you then?"
Then, you say your good-byes, hang up the phone and realize that it's over. You made it, dude. You just talked to a girl on the phone and lived to tell the tale. Doesn't that feel awesome?