I have read somewhere that when you get the number of a girl you DON'T call her the next day. If the girl is interested in you you wait about 4 days, then you call her. If she is not that much interested you wait 2 days.
The reason of doing that is maybe the cat-string theory. When you get her number she expects to call her next day, and if you really do it, it would be easy and predicted, so her interest is not going to grow but may be lower. If you make her wait for 3-4 days, she will start wondering why you are not calling her and you will drive her crazy. So when she looses any hope that you will call her... Ring-ring-ring... Hello? OHHH, HI

......
and all that hope is now going back.
I dunno guys, were you in situation where you wait someone to call you. It doesn't matter if it's girl,boy, some important call or even when you're waiting delivery. And you know that they will call you these days, today - nothing, next day - nothing, next day-nothing. And when u lost any hope, Ring-ding-ding all of sudden a great joy is coming to you and all the happiness and all the hope is coming back to you.
I dunno what PUAs here think about this.