Voicemail

You are addressing this to another user and not to Apple in this forum.

Apple Support Community Forum

This is Apple Support Community forum, primarily for a user to user interaction for information & idea sharing between users, if need be, extend support on technical know-how in handling Apple products from a more experienced fellow user, This is not the right forum to discuss with Apple Inc.

Voicemail is never stored on your iPhone, but on your carrier's server.

An example here from my carrier: unheard messages are stored for 8 days. Messages you've listened to will be gone after 24 hours. Longer storage is also possible. Press 5 while listening to your message and it will be saved for 7 days.

The voicemail settings are controlled by the carrier, so contact your carrier.

VoiceMail (Visual or Normal) is a Carrier Specific feature. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you.