Adding a Privacy Option
You could do this fairly easily with the Contact Layout Editor extension (assuming you're on CiviCRM 5.x).


Use the extension to remove the existing Communication Preferences block from the Summary page.
Build a custom block that includes the normal communication preferences fields plus "Do Not Voicemail".
You'll probably want to remove "Do Not Voicemail" from the custom field block in a similar manner so it doesn't appear twice.