Why not shorter than 20 seconds? The voicemail indicator will tell the contact how long the message is, and if it’s just 5 or 10 seconds, it may seem like a spam call or a wrong number, which means they won’t listen to it at all. Messages between 20 and 30 seconds seem important enough to listen to but not so long that they’re off-putting for the contact.
I find it helpful to imagine that I am writing a memo when I leave voicemail messages. Outlining points enhances my own clarity and puts the listener on notice for the information.

When you have new voicemail, the Phone tab in the Skype for Business main window displays the number of your messages. See Contact Card opens the caller's contact card, which lists their phone number, email address, office location, and so on. Open Item in Outlook provides more information about the call.
If you’re contacting the same people with marketing voicemails and emails, you shouldn’t repeat information verbatim. By mixing up your approach, you make it more likely to get a response to at least one method. Also, your voicemails should be more personalized than your emails. For example, you could send an email to a contact – and 500 others – asking for feedback about a specific product they recently bought.