There is no way to determine if a voicemail is legitimate , or not. You can blacklist phone numbers, but Robocallers rarely use the same number for long. If they block their number (anonymous), then you may be able to block them. If any voicemails are less that a certain length, a few seconds, there used to be, and probably still is, a minimum length threshold that could be set, to reject the message altogether.
You could set up an IVR (Digital receptionist), to "screen" calls before they reach employees. "Thank you for calling XYZ company, please press 5 to speak to the receptionist, or if you know the number, dial the extension now". This should block them, as, being a recording, they can't respond correctly.
