I believe you need to disable email notifications for faxes? if that is what you mean by blank voicemail?
pic below shows where to find them if you are using a user ext.

 

So, a fax will come in to the number, but under “Phone” there is a voicemail message that is dead silence. This happens every time a fax comes in, and the voicemail messages are of various lengths. The Fax shows up digitally under the “Fax” portion, as it should, but they are always accompanied by a voicemail message with no sound.

sounds like a bug. 
You could try one of the following to see if it affects the behavior:
1. in the app > Settings > Phone > Incoming call rules > Incoming calls > If no one answers > Play announcement

This will remove the voicemail for the user, are you still getting voicemails generated?

2. what number are you using to receive the fax?
a. Company fax number
b. digital line/ direct number of user extension
c. additional number assigned to a user

if it’s b) or c) you could try changing its type to “Fax only” as opposed to “Voice and Fax”. To test if that gets rid of the voicemail.

The most important thing is that whatever testing you do, write down everything that happens - something like this:

Fax1
From: 012345 12345
To: 034567 55555
Date: April-18
Time: 14:24
What happened: Fax received, silent voicemail (25sec) also received. Two email notifications received - 1 for fax and 1 for voicemail.

Fax2
From:
To:
Date:
Time:
What happened:

and so on. Then raise it to RC support.

To answer your question in the original post:
Am I missing something? 
No, there isn’t a setting about generating a voicemail when receiving a fax. Should not be happening imo.
 

For documentation, the issue was resolved. I changed the extension to Play Announcement instead of sending it to voicemail and the voicemails stopped populating. RingCentral support answered my ticket after this change had been made and had me put it back to sending to voicemail. So, to clarify, the setting was changed, and then changed back to the original setting, and the issue is not happening anymore.

I believe you are correct with it being a bug. Unfortunate, but it is resolved. 

Also, you are a saint for replying in the way that you do. It is people like you that make these communities actually useful. Thank you.