When you make a call and get voicemail - don't just leave a garbled message as you rapidly try to think of something to say.
Speak slowly and clearly. If you leave a number repeat it. It's often difficult to write down a number in one go, especially if the caller rapidly rattles off their number. Repeating it gives them a better chance of getting it right first time.
State the nature of your call - if appropriate. What are you calling about, what information do you need?
Say when you're available or when it's a good time to call back. People will often leave a message just as they're going out - usually as a last minute, 'oops must do this before I leave'. When the call is returned the person isn't there and you end up playing phone-tag.
If you're going to be away from the phone or are busy - say so in your message and say when you'll be back, so they don't waste their time or yours.
(Photo credit: Wikipedia)