Voicemail service is in addition to your regular telephone service. There is a $2/month charge added to your phone bill, so the request for adding Voicemail to your account needs to come from your department's Administrative Assistant.
If you have a Message Waiting indication (stutter dialtone on an analog phone or MWI light lit on a DTerm multi-line phone), you already have a Voicemail Account from a previous user. Call the Help Desk at 7900, give them your name and campus number and request the Voicemail account for it be reset.
If there's no stutter or MWI notification, call 7590. If you get a recording asking for a Security Code, you have an account from a previous user. Call the Help Desk with your name and campus number and request a reset.
If you get a recording saying "If you know the extension", then there's no Voicemail account on your number. Have your department's Administrative Assistant request a Voicemail account for you.