One thing I really get frustrated with is unnecessarily long voicemails. Here's how I leave a voicemail:
Step 1: Say "hello" and the recipient's name.
This helps to verify for the recipient that this message is in fact intended for them. It's not foolproof, but I'll get to that in a second.
Step 2: State who I am.
Unless I'm calling the person on a very regular basis, I never assume the recipient has my number or can recognize my voice. And it helps to avoid putting the recipient in the awkward or rude position of asking who you are. In fact, a couple years ago I got a message from someone asking for Lou needing help with his computer. I helped the person attempt to bypass their blue screen of death for 20 minutes before I just couldn't help myself anymore and had to ask him for his name. Turned out, I didn't even know the person. It was a complete stranger who dialed my number by accident and just happened to be calling for another guy named Lou! And for that reason . . .
Step 3: Use my last name.
See story in previous step.
So far: "Hi Mark, this is Lou Abramowski"
Step 4: State the purpose of my call (quickly)
This is not story time. Simply state why you are calling. Do not go into some long explanation or summarize any data charts. Make one point, maybe two. And if you have never spoken to the recipient before, you should preface the purpose with how you got the phone number: such as, "Matt Ellsworth referred me to you and told me you could provide information about . . ." or "I got your number off of your website."
Step 5: Set expectations
State whether or not you want or expect a call back. If you do . . .
Step 6: Say my number . . . SLOWLY
Read your phone number slow enough that someone can write it down.
Step 7: Say my name again
Remind them of your first and last name again -- in case they weren't ready to write down notes when you first said your name. It's worth stating your name slowly and clearly here to ensure they understand you and to give them time to finish writing your number, your name, ... as well as to give them time to confirm when you ...
Step 8: Repeat my phone number
Yep, maybe it sounds stupid, but just give the recipient a quick chance to confirm what they (presumably) just wrote down.
"Hi Matt, my name is Lou Abramowski and I got your name from my friend Kris who told me you make websites. Well, I need one and could use some more information. If you could give me a call back, my number is 612-384-0679, I'd really appreciate it. Again, my name is Lou Abramowski and you can reach me at 612-384-0679. Thanks Matt!"
Incidentally, my voicemail greeting currently speaks "Hi, this is Lou, and I like short messages" and is rather counter productive. Turns out, people find it especially amusing to leave me really long messages after that bit of coaching :)