I agree with what Carrie said, but add in that the message must be left with a professional voice, slowly and clearly.

And I've got a warning of what to NOT say, 'I will get back to you at my earliest convenience"... so many people make the mistake of saying this and it is actually backwards, because the client is supposed to get a return call ASAP, not at your convenience, which basically means, when you get around to it...

call me a stickler...

have fun and good for you by focusing on the little details like this!

Definitely! And, when you record your message, make sure you are doing it in a quiet place. A very professional voicemail message can be ruined by hearing a lot of people talking in the background, or a baby suddenly screaming (yes ... I have heard this). I have even heard a voicemail message where the person was shouting over background noise. Just the fact that you are thinking about how to make a good message tells me you are probably already doing it better than those who don't think about these things at all.

I have to say it took me longer to get back to this than I anticipated but whoohooo :D  I finally did!  Professional voicemail script recorded: CHECK. :D