Interest level. Plain and simple. One call, leave message, on to other business you go.
I was just jacking around with a chick online and got her number, made the call, left a message, and left it alone. Nothing further.
I hadn't heard back from her for a week, then today, an e-mail message. "Hello... How have you been?"
Now, knowing all that you know, what's the appropriate course of action now?
A. E-mail back?
B. Call her?
C. Do nothing.
D. Text?
E. Come to the forum and ask everyone's advice?
The correct answer is C. Do nothing. She has my phone number. If she was interested, she'd call back. If not, she's an attention wh0re who just wants a pen pal to chat with and kill time while at work (or just not interested and looking for some entertainment). Period.
There is an article about "the dramatic pause" where you ask a question and then shut up until you get the correct response. Chicks will back-peddle and start to change their answers in an attempt to qualify themselves to you. This is a similar scenario. By me not calling 2 or 3 times, I not only demonstrate that I'm not desperate, but she'll sit and wonder what I'm up to, why I don't talk to her, what she did wrong, whatever... she'll run the gamut of emotions on her own. Meanwhile, I'm wasting my phone calls with chicks that put out, don't play silly games, essentially spin the other plates. It will infuriate her to the core when she realizes that I might be straight-up rejecting her (which I probably am)... then she'll start chasing me: as it should be.
Then again, I don't tollerate disrespect. And, I categorize "not returning calls" as disrespect. Furthermore, I hate talking on the phone to begin with, so talking on the phone to an answering machine really fries my noodle.
Game-playing, crazy chicks are a dime a dozen. To waste more than 6 calories on them wondering "WTF?" is a shame. Women are not your priority, talking to women on the phone is not the basis of your happiness, chasing is a no-no, I can go on and on.
One call, leave a message, and get on with your life. Don't give the woman a second thought until she calls back. "Only takes a minute"? I could use that minute to call a woman with a higher IL.
Also, keep in mind that a woman will sometimes give you her phone number just to get you out of her face (as FD'A mentioned). Which is why many advocate on-the-spot escalation versus getting a phone number and bailing.