After taking a look at your account I can see you indeed do not have Voicemail settings enabled on Commportal. So, your voicemail offered by Sonic is already disabled.
The best way to essentially "disable voicemail" since your phone system has a built-in answering machine, you're using would be to change the call settings in Commportal.
You would need to first access Commportal by navigating to https://members.sonic.net/
and logging in
Once logged in follow the steps below
1. Click the voice tab on the left-hand side of the screen
2. Click the phone number you would like to disable voicemail for under Phone Number List
3. Click on Notifications
4. Uncheck the box next to "Send phone notification of incoming messages to the following phone numbers"
5. Uncheck the box underneath "All Voicemail" for the associated phone number
6. Click Apply to save the changes
The steps above should disable the Voicemail noise notification you are referencing.