After hearing that first message, she has no real reason to call John back. He didn't say anything that was unique or even interesting, and he didn't give her any reason to look forward to talking to him. And that's the second principal of getting callbacks.
Second principal of getting callbacks: Help her look forward to talking to you.
Think about it. What do you want her to be feeling when she's thinking about calling you? Do you want her to just put you on her
list of things to do... you know, like her laundry, and taking out the trash?

Or, would you want her to actually be looking forward to speaking with you. Do you want her to feel a little bit excited, anxious, and yet forward looking and happy... all at the same time?

Well guess what... it only takes a tiny bit of thought and effort before you call her in order to get her to feel that way about returning your call.
But before we go into that, let's figure out what the problem is.

Exercise #1: Write down the message you usually leave a girl on her answering machine. Actually get out a pen and write down what you say.

Exercise #2: Call your own answering machine, pretend it's a girl's machine, and leave a message just like you would if it was a girl you just met, and you wanted her to call you back.

Both of these exercises are very valuable for discovering just how DAMN BORING your messages probably are. But it gives you a place to start, and besides... you can't change something and make it better until it exists. By writing it down, and recording yourself... you're bringing it out in the open, so you can look at it objectively.
After you do both of these exercises, listen to your message and read your writing... but pretend you are a beautiful girl who just got 10 other messages from guys who you. Now... would YOU call you back?

Also, do you know what the number one thing that will make a girl NOT want to call you is? It's needing her to call you. Sounds simple, doesn't it?
Have you ever been around someone who was like an emotional vampire? Everything they did or said exuded neediness, and you just didn't want to be around them. Well the same is true for women and dating.

If the tone of your answering machine message suggests that you REALLY want her to return your call, she probably won't. It's a quirky little fact, but I'm sure you've experienced this in your life. The easiest way to make someone want to give something to you is to really NOT need it.