Voicemail

As Lyssa says, contact your carrier.

Here are a couple of troubleshooting steps you can do yourself:

• Make sure you have Cellular Data turned on. Visual Voicemail cannot use Wi-Fi, because your carrier doesn’t have access to your Wi-Fi connection
• Try restarting your phone→Restart your iPhone - Apple Support

Beyond that contacting your carrier and asking them to reprovision your visual voicemail is the next step. Visual Voicemail is a way to transfer the contents of your carrier’s voicemail server to your phone. But the actual voicemail itself is recorded, stored, and managed on the carrier’s voicemail server, not on your iPhone. The VVM function on the phone is an interface to control that backend service.

Are you asking about Visual Voicemail? That is a carrier feature. Have you contacted them?

~Lyssa