If you have unlimited login attempts to the voicemail system then why bother "proving" anything? If they've set it up that way it's a FACT that it can be done. If the company can't see that then please tell us who they are so we can avoid them.
Further why would it matter what the originating caller ID is? Shouldn't you be able to login to voicemail from any other phone? If you limit the phones from which an end user can access their voicemail that kind of limits voicemail's usefulness.