In the admin portal, go to User List > Users with extensions and find the fax extension. When you click on the extension, you will have a toolbar menu right under the name with an option for notifications. Uncheck the email notification on the Fax item.
I just read your message in a different way. If you don’t want the caller to ever hear voicemail, make the line as a fax only line. You do that by pulling up the number from inventory and under number type, select fax only: