How do I find the "Mailbox number specified by your system manager"

I have a new iPhone 5s and set up email and a password initially, but realized after a couple weeks I am not getting voicemail. When I call from another phone I get a message that "the party is not available"

When I call my voicemail it asks to please enter the "Mailbox number specified by your system manager"

I have tried my phone number, my last 4 digits, and my pass code. Tells me it does not recognize the number.