The answer is: it depends on the impression you made on the girl during your approach. A girl will only give you her number 'to get rid of you' if you gave off a bad or creepy vibe or something.
Instead of worrying about getting fake numbers, get to the root cause of the problem, which is YOU. Work on bettering yourself and bettering the vibe you project when you approach a woman. Before you approach a girl, ask yourself "How can I brighten her day a little?".
Basically, if a girl gets good feelings from being around you, she is far less likely to give you a flakey number.
One thing you're all missing, though, is that you should ONLY get a girl's number as a last resort. You should try to get to go for a cup of coffee with you there and then. If logistics are not possible, for whatever reason, ONLY THEN do you settle for the number.
I've found this a better and more efficient approach.
Hope that helps.