Well, the deal with flaking is that you didn't nail your intial interaction with her. Ideally, you don't want to get her phone number, you want to set up your first date with her. If you can't get her to go out with you while you're standing right in front of her, there is no way you will be able to land a day two over the phone.
Usually when a women gives her phone number and flakes it's because of: not enough rapport. Typically, she will be attracted to you, but you weren't special or different. Attraction for women work likes attraction for men. Its instant and fleeting.
Think about it. When you see a hot chick, you are attracted right away. As soon as she disappears from your sight and another hot chick comes across your path, you have already totally forgotten about the first girl. Instant and fleeting. Most guys make the mistake of assuming just because she's attracted that she'll want to see you again. You need to be different and special. And I don't mean like DHV special.
You need to actually connect with her in a way that no one else has. You need to understand her in a way that no one does when they first meet her. Then she'll remember you and think about you and be waiting to hear from you.
Also, as soon as a I get a phone number I do two things. I call it right there. I have made a little routine out of it. Dial the number and stare right at her. If she doesn't get it, tell her answer your phone. Then its role playign time...
You: Hey, whats going on? So I got this really important questions that I need to ask you.
Her: Oh yeah?
You: Yeah, I met this girl in the bar last night...she was wearing(describe what she is wearing). Anyways, I know its kind of weird to meet people in bars, but she was different because (desribe why she was different). Anyways, I don't know if I should call her though. What do you think?
Her: Whatever...
You: Yeah, I don't know, I don't like wasting my time. She was cool, but she might be this weird stalker, you know what I mean?
You're basically making the first phone call this fun little game and setting up some hoops for her to jump through. The key to phone numbers is you always need to reverse teh frame. You might be asking for her number, but you cancel in heartbeat if you think there is the slightest chance she might flake.
Like mentioned above too...I always stay for at least another 5 minutes to shoot the shit with the girl. When you get her phone number and leave, it feels like a pickup. Girls don't want to be picked up. They want to meet cool guys.
If you're not day 2 closing girls, you'll be dealing with a good amount of flakes indefinitetly.
Later,
Joe