Disable voicemail. This should frankly be the default anyway - voicemail is a relic of the past. Nowadays we have SMS or email for those who want to leave a message.
Alternatively create your own voicemail (using Twilio or Asterisk or similar) and change the call divert settings to divert to your custom voicemail instead of the default one.
Can an attacker turn it back on thru the automation schemes discussed. Hopefully not..
Doubt it but always worth trying these attacks yourself to make sure.