To access your voicemail dial:
*97 - from your extension , then your extension password
*98 - from another other extension on network, will enter your extension when it asks for 'mailbox' then type the voicemail password
The voicemail password and setting can me managed from your freePBX admin panel:
>Look up extension to manage (found under > Applications >> Extensions)
>>click on 'Voicemail'
>>> after you make changes, CLICK "APPLY CONFIG" in top right side of window