Select the switch next to Allow callers to leave voicemail. When enabled the switch will be blue and positioned to the right.
Select Save.
Additional Information
Enabling this does not notify the caller that the option is available. At least one of the audio files (Initial Queue Greeting, Music-on-hold, or Repeating Queue Message) should be modified to include the notification. Usually this notification is contained in the Repeating Queue Message.
For more information, see: