Well, if you ever want to have a reason for not being able to take a particular call, AT&T gives it to you. I especially like their "You have a voice message, but we're not going to let you listen to it" feature. It is a nice compliment to their "Oh, this message came in several hours ago, but we had better things to do than, you know, actually sending it to you" feature.
I personally am a fan of the you have full bars, but no service, or the infamous deadzone on the K street bridge over rockcreek park
