I have not responded to one of these in awhile, but your problem is pretty clear cut. And it's been nagging me that the nail hasn't quite been hit on the head. The issue is that you're basically asking a chick for her number, just to have her number.
A girl doesn't want to just give her number out to some guy online. You need to give her a reason. My recommendation? Skip the number and go straight for the date. First, it shows confidence and we all know that confidence is a PUA's currency. Secondly, and more importantly, it gives her a REASON to give you her number. (keep in mind, if you get the date and no number, she will likely forget about you and flake)
I mean, "i'll call you sometime..." why? for what? These are the questions she's gonna ask herself, and then she will undoubtedly find a way to think of you as weird or creepy (no reflection on you man, that's just the average girl's psychology).
However, if you're smooth and get her to agree to have a drink with you (which you will be surprised to find out is much easier than getting her to give you her number just to give you her number), the number becomes kind of an after thought, like "oh yeah, i should probably give you my number so we can coordinate this".
Online is my main domain, and it's one i really like because you can experiment without feeling awkward about how the situation is going. I've found that dates are easier to get than numbers (and usually lead into the girl giving you her number). If you're good at this, and the chick isn't an idiot or hesitant about you, you shouldn't even have to ask for it... She'll just give it to you as a "common sense" move in orchestrating a date. Keep in mind, Chicks think differently than we do. You always have to give them reasons to do things, even if it's something that they already want to do. it helps them rationalize.. Think of it this way, you have to give her something to say to her best friend when she's explaining WHY she went out with/ likes/ slept with you. You wouldn't call a chick and say "Want to come over and have sex?", but you might say "Hey, want to watch a movie?" You both know what that means. But, she would feel like a whore if you didn't put it more subtly and this way when she's explaining it to her friend she can say "we were watching a movie and it just... happened." instead of "He called me up for some butt, so I ran over.."(which she will not say and therefore will not do...) Hopefully, that illustrates the point.
So, When I ask a girl out I always say "wanna catch a drink sometime? we can talk about or do so and so" with so and so being a reference to some humor I seeded early on or something common to the two of us. That way I'm not communicating "I want to spend time with you" (to which she will think,' why? you hardly know me...' which, in turn, will cause you to dlv), I'm communicating "I want to talk about so and so..." which she can rationalize as a normal and pressure-free activity. It's a subtle and seemingly insignificant move, but it makes all the difference....
Anyway... I may have deviated a bit from the point which is 1- Give a reason when asking for something 2- Don't make more steps than you need- Go for the date. If that's a yes, the number is basically implied.
Hope that helps man