First, go to the "My Direct Line" page in the left-side navigation.
To record a custom voicemail greeting, simply click on the "Record Audio" button. This will use your computer's mic to record a custom file. Select "Begin Recording" and you are all set!.
Be sure to hit "Save" before exiting the recording dialogue box.
This newly recorded file will be available in your Better Voice account to be used on My Direct Line or any Extension if needed.