You may be able to help those who ligitimately want to get hold of you. Your message could say something to the effect:
"This voice mail is not in service. Please send a text to same number if you need to leave a message"
On a related point, I cannot find any Knowledge Base article or something similar that outlines all of the various VoiceMail functions and features. Is there one?
If anyone is discovering this thread and wants to know how to set up the extended absence greeting and prevent people from leaving you mesages, I outlined the steps here last week: https://productioncommunity.publicmobile.ca/t5/Getting-Started/Re-Disable-Voicemail/m-p/213805/highl...
A couple of alternatives: