I can't come to the phone now because alien beings are eating my brain. Leave a message anyway, and after the alien beings assume my shape, one of them will get back to you.
I'm unable to take your call in person because I'm having an out-of-the-body experience. In fact I'm standing right behind you and I can hear everything you say. But leave me a message anyway to help me reconnect when I get back.
Steve is reassembling Elvis' brain and can't come to the phone right now, but if you leave your name...
Hi, this is Jim. Sorry I can't take your call but I'm playing my guitar too loud to hear the phone ring. Please leave me a message and I'll call you back at the end of Van Halen-1.