In order to access the voicemail setting, click on the "Voicemail" tab (as shown in the picture below)
These are what the options within the voicemail tab will do:-
Voicemail On - Used to turn voicemail off and on
Voicemail Code - Used to enter your voicemail code
Voicemail Email - Used if you want your voicemails to be forwarded to an email address
Voicemail to email notification - Used if the user wants to be notified by email if they have a voicemail
Voicemail Ringback - Your telephone rings whenever you end a call and replace the handset