Like Mystery says number closes don't mean a thing unless you've time-bridged another meet. People (including myself) place too much emphasis on getting a hot girl's number and when I have done, I feel like I'm walking on air!
No here's the problem, you can't fuck a phone number, lol!
Any which way mate, perseverance is the key! I kiss-closed an intern at work last Friday but I had to do some serious "chasing". I think the trick is even though you know you're chasing them like a bastard...you don't want to give the impression that you're chasing them.
I think the worst thing you can do is let a trail go cold unless she explicitly says, "Leave me alone". What I've done is text them random stuff from time to time. Female opinion openers about getting flowers for an ex-gf.
You need to find a way of differentiating yourself from every other AFC in her life that's texting her which means working the "pre-selection", "having better things to do" and "fun & interesting" card a bit more.
Keep at it is my advice mate! No guts, no glory as someone once said. Good luck
