First of all, the most important aspect probably contributing to your phone-flakes is the style of
message you're leaving. Let's assume your initial interaction was solid and you developed a good amount of attraction. You have her phone number. So, logically, all you have to do is make a good call to have her out on a day two. If you call and get the machine,
DON'T remind her of who you are or how you know her. You're the trophy, you assume that she's going to recognize your number and your voice.
DON'T leave a long message. Make sure that your vocal tonality is neutral, not supplicating and striving for rapport. Your message should come off pretty nonchalant.
When I get a machine, my messages are always pretty simple and short. I'll often throw in a little neg or something totally random and ridiculous, something fun that could plausibly have entered my mind out of nowhere that made me think of her, and then I'll elusively mention something about talking later.
Ie: "Hey, you. I thought I just saw you in town, but you got closer and I realized you were a guy. Rough. Anyway, I'll all over, so try to get a hold of me."
I assume she knows who I am, because I'm the hotness. I talk as though I'm speaking with a little girl that I'm superficially kind to, though she annoys me just a little bit. I always make sure to include a glimpse of lightness, a little laugh, a slight raise in intonation, kindof like the vocal equivalent of a smirk. I don't ask her to call me back, I assume she will. Not only that, but I acknowledge subtly that it will be difficult for her to reach me, implying that I'm a busy, high-value guy.
Most of the time, I get call backs within the hour. And, funnily enough, I don't have call display, so I often get girls confused with each other. They get the impression that they're not the only girl in your life. Awesome. It destroys the perception of desperation on your end.
DON'T try to make dates on an answering machine. Get them to call you back, and when they do, then you spring the hangout on them, you don't ask for it. Something like, "OK, Let's go shopping. I need a new slip-and-slide. Come on, sko." Be decisive, tell them what you both will be doing, and plow through. There is magic in "SKO, SKO" (Let'sGo, slurred).