Welcome to our forums and thank you for getting in touch. This trouble that your relative is experiencing with their voicemail certainly sounds frustrating.
Could you please confirm whether this is the Virgin Media voicemail service that's included with the landline, or is it a separate function on the handset itself? If this is our service, we can certainly look to have this deactivated. Just bear in mind that any attempt to retrieve previous messages would usually reactivate the function.
We'd need to clear account security in order to make these changes, is this something you'd be able to do?