If you are new to a school, classroom, etc... check the name one the phone's display. If its not your name, let us know via a helpdesk ticket and we will get that changed.
If it is your name, then we've made the change already, but you need to setup your voicemail. Press the voicemail button on your phone and use the default new account passcode '1234'. You will login and follow the prompts to setup your voicemail and pick a new passcode.
Thanks!