@Triguy 's link is a good solution, as it also allows you to have a personalized message without ability for callers to leave a message.
You can also completely prevent calls from going to the voicemail service at all by setting all conditional call forwarding to an invalid/US number by dialing *004*8080000000#
Now declined, unanswered, and calls while out of service area will hear a fast busy or endless ring instead of public mobile voicemail.
Revert this to default PM voicemail by dialing ##004#
Edit: hehe @gpixel beat me. 🙂
