You know that your callers are making inferences about your business and brand. If you want your voicemail recording to leave a positive impression, smile when you record it! Research shows that smiling affects how we speak, and listeners are not only able to identify that people are smiling, but also what the intent is of the smile based on voice intonation alone.
If you can, try to keep your message 20 to 30 seconds long — max. This is the sweet spot for sharing just enough information without going overboard, potentially getting cut off by the recipient’s voice mailbox system, or having the listener hang up early because your message was simply too long. Speak clearly, avoid rambling, and get straight to the point.

Whether it’s your last encounter or a recent e-book download, have a relevant reason for calling in order to get the prospect’s attention.
Repeat your phone number twice. People seem to forget that the receiver of their message has to write the number as you say it. Don’t rush through it. Even when you say it slowly, it’s hard to get down the first time. So repeat it again, so they can check to be sure they got it down right.