Everything the above two posters said is correct.
HOWEVER. You gotta understand that if you randomly get a chick's number and call her a few days later and ask her out on a date, it is MORE THAN LIKELY that she will not want to meet up with you. It's easier to give out her number than to reject you. Because when she gives it out, you go away. Unless she is REALLY into you, you'll almost always fail. Which is why the whole "it's a numbers game" thing is so convenient. Because if you repeat this process with 1,000 girls SOMEBODY is gonna want to fvck you, and you will get that date.
However, just because a girl isn't all that into you when she gave you the number, it doesn't mean all hope is lost. She may be SOMEWHAT attracted to you...just not enough to waste her valuable time going out with you. So, now what?
I've found it particularly helpful to game these girls a lil bit via text. There are lot's of great posts out there on how to do it. Basically, you're building a little bit of rapport and attraction over a few days/weeks. Don't be a chump texting her everyday about boring mundane stuff. Do it strategically. Random, funny texts. Sexual innuendos after awhile. Get her thinking about you. And THEN you push for the actual date. Usually just texting for this is fine as well.
Now for the disclaimers. No, this doesn't guarantee she still won't ignore you, but it definitely improves your odds. And it's still a numbers game, so keep finding new prospects.
Also, a woman's age is directly associated to how successful you'll be. If she's 50, she probably isn't big into texting. If she's 20, it's probably her world. Know your market.
A lot of guys say texting is for pvssies, but if done correctly, you will have great success. I only text girls, and it works for me. If you're under 25, you pretty much will HAVE to do almost all of your game via text (other than in person, that is).
Happy hunting.