"if you're passed over for a job, do you mind being notified via voicemail or email?"
My answer is plain and simple : "neither, call them!"
My opinion on this one is very strong and for several (I think) very good reasons. Allow me to explain :
You are the hiring manager. you're the person who the new hire will report to. You're probably also the one who will lean back, balancing your chair and sipping from your nice glass of Shiraz while you say, I quote, "I'm the one that hires and fires." (blank stare goes here).
By delegating bringing bad news to a direct report or, worse, someone in HR you have showed that you are only willing to be responsible for your hiring decisions and not your firing decisions. At the same time you have passed on the chance to take ownership of this decision. You have reduced your candidates to resumes that you pick from and not the actual humans that will bring value to your team. By not committing to stand behind your choice, you actually do yourself, your team and your organisation a disservice.
Now, I do understand you are a busy person so the first argument I will hear when discussing the practice is "I don't have the time to call everyone". I call bullshit.
If you're THAT busy you will have your hiring process geared to your agenda. You will be choosing from 2 to 5 candidates that you will have personally seen. Those persons that didn't make it will take less than a minute. The chance that they are willing to strike a lengthy conversation is zero. Even though they won't realize it, they will appreciate this gesture in the long run. To the extent that, if a similar position opens up they may even be enthusiastic enough to consider working for you after all.
In the end you're looking at less than 10 minutes of your time to call ALL candidates (including the one you do chose to hire).
Note that I've been on both the dealing and the receiving end of this practice. I know that it's not easy to tell someone they're not hired. I also know there is no good way to receive that information, especially if you've thrown yourself passionately at a job opportunity. It fucking hurts.
As a manager, telling someone they're not hired is peanuts. You just gather yourself and tell it. Look at it this way : at least it prepares you for the really hard decisions. If you can't even tell someone they're not hired, how are you ever going to handle firing someone?
Now grow some cojones and stand behind your decisions. That or don't call yourself a leader while I'm near you ;-)