rule of thumb.. if you're a guy.. you can't call her back until the 2nd day after she gave you her number..
now, as for what kind of sign it is.. women give their number to guys for two reasons:
1. To get rid of them and make them go away.. (usually a fake number)
2. To have the guy call them (which is still a test).. they'll be turned off when you call too soon, and turned off when you call too late (soon being anywhere within 24 hours of getting her number, late being more than 3 days after getting her number)
Now, one thing that put me kind of "off" was when she said she was busy..
"Busy" is analogous to "let's just be friends".. allow me to explain:
I've had a "Nursing Student" give me her number, & a "Medical Student" give me her number.. needless to say, both were jam packed with work, and the first time we met, they were telling me how they both had their standardized exams comming up (aka. alot of studying)..
Now, I never ask for numbers.. so they offered.. and instead of accepting and jotting it down.. I just say "relax, later, i'm too lazy to write it down now".. so they keep finding ways to give me their number.. as they feel i'm about to leave.. so finally, I give them my number, and ask them to call me cell (their number shows up, and I press save).. both said "i'd love to go (talking about something conversationally relevant as an excuse for me to call them)".. and one said "call me"... NONE of them said "i'll be busy, but here's my number".. THINK ABOUT IT! these are women who are jam-packed with work, and they didn't throw the whole "i'm busy" crap my way..
So, if I was you, i'd throw some of her crap right back.. call on the 3rd day (don't be a wussy and text; CALL!) and tell her hi.. casually slip in the fact that you were so busy yesterday (but not for all 3 days, just yesterday.. to have her thinking that you COULD have called the other days, but you didn't.. and maybe this "busy" is just an excuse.. but "oh wait"! she can't call you on that because she's also guilty of throwing that crap your way)
WARNING: don't call women on their crap.. it makes them feel uncomfortable.. it's all well & dandy if you're on top of things and know exactly what's going on, but keep it to yourself.. mainly because if you make her uncomfortable, all she has to do is stop talking to you.. but secondly.. if you make her uncomfortable and she still talks to you, you give her a head's-up on making her tactics more complicated, so never give her information about how much you KNOW she's pulling on you.. keep it to yourself and let her believe she's fooling you if it means she won't feel uncomfortable.. (let her believe she's fooling you, but at the same time, don't let her feel like she can manipulate you, there's a fine line, be sure to not cross it!)
If you can't stop the Wind, then you can't stop the Storm.