Here's a tip: When you call someone at their workplace and don't leave a message, chances are, they don't know you have called. If you are like me, your phone isn't some super-powered high tech phone where you can see all your incoming call logs. It's a desk phone, not an iPhone.
Deductive reasoning: THEY WON'T CALL YOU BACK IF THEY DON'T KNOW YOU'VE CALLED.
Why this drive me up the wall: When I get a phone call and the person says to me, "Boy - you are hard to get a hold of!" Then they chuckle. I say, "Oh, I'm sorry, I didn't get a message from you," it's sincere this time. "Well, I didn't leave one. You must be realllllly busy," They say. Oh, come ON! How in the world do you expect me to call you back? I am on the phone frequently, take bathroom breaks, eat lunch daily (that never fails), have appointments that are not in my office, and also am on the phone often. "Oh. well, my mind reading machine is in the repair shop," sarcasm's thick on this one.
Moral of the story: Don't just assume that you can catch someone on one ring. Leave a message. Send an email. If they don't get back to you, shame on them, but give them the opportunity to at least try. Don't just ambush them as if they are not doing their job!