If a girl is really into you, and you give her your number, she will get a hold of you. If she takes your number but was just being polite, you can avoid wasting time on flakes. If she wants you and she has your number, she'll call or text. I think it also shows confidence on your part. You don't need her number, because you're confident that she'll call you. You're that awesome.

Yes, girls who are timid will not call, but that is NOT who I want to attract. Confident girls will call.