Voicemail Turning Itself Off

I recently discovered that my voicemail settings were turned off via my iPhone 13 Pro Max carrier, Optus. I ensured I turned them back on.

However, today (less than a week later) all voicemail settings were turned off again and I did not do this. I have restarted my iPhone once since turning voicemail settings on.

Has anyone else had this? Surely my voicemail settings shouldn’t reset themselves every time I turn my phone off! How can I stop this from happening?

Thanks.