You ask for her number. You don't explain why. She knows why you are asking. If she asks why you want it, she's trying to avoid giving it to you.
If you tell her you want to "get to know her" you will look lame and put some pressure on her, reducing your chances.
You can ask for the e-mail after she has given you her number, but don't ever bother using it.
If you see her all the time you can ask her out in person. You will avoid the hassle of missed calls, answering machines and messages. Simply suggest a date, time and activity. Let her say yes or no. If it's not a yes or counter-offer, it means no.
Don't put obstacles in your way. If you see her in person you don't need to mess about with phone/e-mail. After she accepts your date
however you can exchange numbers (you may have to cancel/reschedule/who knows).
But why let her have YOUR number? For one thing, if she doesn't have it she has an excuse if she flakes without bothering to call.