i've never believed in that stupid rule. "wait two days before calling" is complete bullshit. you're trying to develop a relationship with this woman -- not play fucking games. if you didn't come off as desperate when you met her and got her number in the first place she won't think you're desperate. in fact she'll think it's cute that you liked her enough to call the next day (because she expected you to wait like every other cheesedick motherfucker) and your good first impression (you did leave a good first impression, right?) will still be fresh in her mind. meet girl. make good impression. get number. call whenever you feel like it. if she liked you she isn't going to give a shit that you called "early" -- she'll be happy that you called when you did. go grab coffee, show her your dick, profit.