My guess is, not only will not call you, but she will brag to all her girls about she has some guy by the balls waiting on her to call him.
The reason she asked for your number instead is because she either
a.) didn't have enough rapport with you
b.) thought you just wanted her # as a trophy. there are guys out there that will approach a girl, get her number to feed their ego and never call. it's happened to almost every girl, which is the reason this one may be defensive about giving up her digits.
c.) isn't interested in you. (usually the main reason)
To avoid this mess, always establish lots of rapport first of all. Instead of just throwing the "what's your number" at her, ask her out for an activity first. Tell her you it was great meeting her and that you'd like to take her out for coffee and get to know her better (or whatever else you think is a good idea) sometime. If she agrees, she will give you her number willingly most of the time, and if she's not, then at least you know then, and don't have to wait for her to keep flaking on the phone.
This way, not only do you establish rapport, but she knows you are interested in HER, and are not number hunting. I've never gotten a "why don't you give me your number instead" with the above method.
Good luck.