It all depends on your provider. If you Google how to turn off voicemail on "insert provider" it should tell you what to dial. Some are a basic number to turn off, some you navigate through a menu.
I would suggest to contact your network provider as they can turn it off or give you instructions how to do so, if you on EE let's say you can send a text message to disable it, also you can open phones dialer type ##002# call and see if it disables it (autoblocker needs to be disabled in settings)