This is driven typically by your network sim card @Dave56
I'd suggest to double check that you've listened to all your voicemails and deleted where appropriate.
If you feel you have then ask your networks customer services to check your account.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.
If you would like to reduce this notification to almost nothing then i have the best solution to permanently reduce this notification
Inside of the phone app go to settings>voicemail>notifications>and silent and minimize them, this hides the voicemail notification unless you expand the notification panel then it there but minimized