If a call is not answered within the No Answer Time, the incoming call will appear in Call History under the Missed tab.
Double click the Call to return the call. (If the outbound prefix e.g. 9 is not shown contact your System Administrator.)
If the caller has left a Voicemail message, the Messages tab in the Call History pane will indicate you have one new message.
Double click on the Voicemail information to listen to your message or right click within the Messages tab and select Call Voicemail.