Visual voicemail not working

SMS, MMS, WiFi Call, Call Forwarding, Call Waiting, Call Merge, Conference Call, Hotspot, Voice Mail (Visual or Normal) all are Carrier Specific features. Contact your Carrier (Cellular Service Provider), the agency that issued the SIM / eSIM to you.

Since that feature is a specific carrier service, contact them and ask for help.

Regards

Giulio