When you are on the phone click the Voicemail/envelope button.
Press 5 to set your current options
Press 1 to change your password

Log into the portal: control.callharbor.com
Click on the Users button

Click the user you will like to change.

Under the profile tab scroll down to the change Voicemail Pin section, you are able to change the voicemail pin under that section, click save after you are done.