Trouble Setting Default Voicemail Message on iPhone After Region Change to USA

Hello Apple Community,

I recently changed my region on my iPhone to the USA to access certain features, including the voicemail feature. In the process, I accidentally set a custom voicemail message. Now, no matter what I do, every time I reboot my device, it automatically reverts to one of the four custom voicemail messages I've set.

I've tried recording the default voicemail message and setting it as my custom voicemail, but it doesn't stick after a reboot. How can I permanently set the default voicemail message on my iPhone?

I'm currently facing an issue with my iPhone voicemail settings. Although I've set a custom voicemail message, the default option is ticked, making it impossible for me to switch back to the default voicemail. Even though I'm using a custom voicemail, the system indicates that I'm in the default mode, preventing me from making any changes.

How can I resolve this issue and switch back to the default voicemail option?