[Rjevski]

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.

[ohiovr]

Can an attacker turn it back on thru the automation schemes discussed. Hopefully not..

[Rjevski]

Doubt it but always worth trying these attacks yourself to make sure.