Do you ever intentionally phone somebody when you know they're not not available to take your call? Maybe you need to just leave a message and don't need to talk about it: remember, the club meeting's a 7:30 tonight. Maybe it's somebody you don't want to talk to: hey, it's your brother—did you mail me that check yet? Whatever the reason, you have to make your best guess about when the person you're calling won't be available, and hope you guessed right.
You can take the guesswork out of it all with Slydial. All you need to do is dial
Slydial is a free service—you don't even have to sign up to use it—but the phone number is in Pennsylvania, so if a call to there costs you, this call will cost you as well. There are extra services available after signing up (speed dial, etc. for your contacts). If you really want to go all out, there is a paid version as well, where you don't get the ad and you can leave messages of unlimited length.