Unless your Channing Tatum or Chandler Parsons handsome... Chances are she's not going to call you. So get her number and GET HER INTERESTED. The best ones are the ones you gotta work for. If they're super easy and they call you imagine all the others that have already been in there.
YOU should get HER number, or the two of you can exchange numbers. Girls will almost NEVER call YOU. Just like girls never approach a guy and ask him out on a date, it's not in their nature to initiate contact with a guy.
for the avg shmuck, there is a bell curve here. meaning he probably has a better chance getting the digits off the 1's-3's and the 9's-10's. midrange girls, the 4's-7's would be more difficult. they're usually insecure about their looks and therefore aspire to have the chandler handsome or athlete or celebrity or wealthy etc by their side to compensate and make them feel whole. the 8's are a curious bunch and actually compose the bottom of the bell curve. hot enough to get attention, but not so ridiculously hot that most guys would be too scared to step to them, they actually get the most approaches and are therefore the most defensive and often avoid new acquaintances. the 8's would be the most difficult catch for the avg shmuck and should just be passed on. the lower tier 1's-3's obviously will give out their #'s or call you back because they're just completely desperate for sex. and the top tier 9's-10's already know they're the shiznit and are generally more open and willing to invite new friends into their rose colored lives. unfortunately for monsieur average shmuck, however, this is usually just as friends without benefits, and often someone who mademoiselle 9-10 can rely on to spill any petty uberfirst world problems she might encounter. a lot of them are bat crazy though and avg shmuck might get lucky for something short term (until she comes back to her senses). but anyways, stay away from those 8's. ok enough for now, end old school ronny rant...
If a girl is interested in you, trust me, she will contact you and do so within a good amount of time. Girls with low interest levels are the ones who will not bother with you. Those are the ones you'll most likely want to waste your time with the most because they are a challenge to you. I've gone as far as to find a way to get her to email me something she mentions or vice versa. Email works in certain situations really well. But phone is better. At the end of the day, staying in touch is a two person game. Girls are not above making a phone call if they like you. It also shows supreme confidence to give her your number and that you don't mind leaving it in her hands. It may even drive her wild when she sees that you could careless if she calls when she sees how nonchalant you are when doing this.
I think it depends more on the woman than how a guy asks. Women usually have a pre-determined way of handling those situations. Some are easy, and will usually give out their number on first impressions alone. Others are very hesitant about giving their number to strangers, even if they make a good first impression. They would rather only take your number, just in case they later decide you weren't that interesting after all.