Now that so many of you have not only cell phones but cell phones capable of sending text messages and email, it has become clear that some of you aren't sure which method to use when. So here's the short course:
When you want to reach someone right away and need an answer right away, send a text message. If the recipient has their phone turned on and with them, they'll get it - right away.
When you want to tell someone something but don't need an immediate response and you don't care if they read it for a few hours or longer, send an email. Most people don't check their email on their phones, so there's no guarantee that they will see your message right away.
If you absolutely, positively need to speak to someone in person, call them and leave a voicemail. But don't be surprised if they don't pick up. More and more, people are starting to prefer texts and emails over talking on the phone.
Examples:
If you are in the middle of Home Depot and have wandered away from your spouse, don't send her an email asking where she is. Send her a text. She'll get it right away.
If you want to let somebody know that you had a really good time at their party last night and ask them for the recipe for the bean dip, don't send them a text. There's nothing that needs urgent attention and you may interrupt her in the middle of something important. At the very least you're going to annoy the hell out of her.
Don't call someone on their cell phone and not leave a message. Unless you have blocked your number they'll be able to see who it was who called them. So don't leave them hanging, leave a message.