Another possibility is that an {extension} was created for that {mailbox} in the voicemail programming.

In SVMi's you can have a mailbox, an extension, or both. If you have a mailbox, but no extension then when a user dials your number the system will dump them right to your mailbox. If you have an extension, when a user dials your number the system will try to transfer them to your phone...if you have no phone it will error and go to the operator.

Try creating a new mailbox without an extension and see if that works.

without being able to log in via a laptop you will not be able to fully administer your SVMi. So if the test box works then try deleting a box that doesn't work and then recreate it as a mailbox only.