You'll need to log into the system via your web browser.
Go to the Voicemail section, then mailbox section.
You probably only have 2 mailboxes.
0 & 9999
edit 0, change your PIN then you can dial into your auto attendant, press *0 to log in.
Having said all that, if you don't know how to log into your system via browser then you'll need to contact your VAR and have them take a look. They need to figure out why calls are going to voicemail if you're not supposed to have it.
Ralph