For my current job, I typically receive somewhere between 80 and 100 incoming phone calls every day. Sometimes, when I'm in the middle of something (like interviewing a prospective employee, printing out some of the 75+ applications I receive every day, or even just typing a document) I ignore a call or four. I promise that I return voicemails, it just might be an hour or so.
I HATE loathe, and despise people who call three or four times in a row trying to annoy you enough to pick up the phone. And then don' t leave a message. STOP! Leave a message the first time you call, and I'll get back to you. Call me 18 times in two days (it's happened, for real), and I'll never call you back again.